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+
+:mod:`mailbox` --- Manipulate mailboxes in various formats
+==========================================================
+
+.. module:: mailbox
+   :synopsis: Manipulate mailboxes in various formats
+.. moduleauthor:: Gregory K. Johnson <gkj@gregorykjohnson.com>
+.. sectionauthor:: Gregory K. Johnson <gkj@gregorykjohnson.com>
+
+
+This module defines two classes, :class:`Mailbox` and :class:`Message`, for
+accessing and manipulating on-disk mailboxes and the messages they contain.
+:class:`Mailbox` offers a dictionary-like mapping from keys to messages.
+:class:`Message` extends the :mod:`email.Message` module's :class:`Message`
+class with format-specific state and behavior. Supported mailbox formats are
+Maildir, mbox, MH, Babyl, and MMDF.
+
+
+.. seealso::
+
+   Module :mod:`email`
+      Represent and manipulate messages.
+
+
+.. _mailbox-objects:
+
+:class:`Mailbox` objects
+------------------------
+
+
+.. class:: Mailbox
+
+   A mailbox, which may be inspected and modified.
+
+The :class:`Mailbox` class defines an interface and is not intended to be
+instantiated.  Instead, format-specific subclasses should inherit from
+:class:`Mailbox` and your code should instantiate a particular subclass.
+
+The :class:`Mailbox` interface is dictionary-like, with small keys corresponding
+to messages. Keys are issued by the :class:`Mailbox` instance with which they
+will be used and are only meaningful to that :class:`Mailbox` instance. A key
+continues to identify a message even if the corresponding message is modified,
+such as by replacing it with another message.
+
+Messages may be added to a :class:`Mailbox` instance using the set-like method
+:meth:`add` and removed using a ``del`` statement or the set-like methods
+:meth:`remove` and :meth:`discard`.
+
+:class:`Mailbox` interface semantics differ from dictionary semantics in some
+noteworthy ways. Each time a message is requested, a new representation
+(typically a :class:`Message` instance) is generated based upon the current
+state of the mailbox. Similarly, when a message is added to a :class:`Mailbox`
+instance, the provided message representation's contents are copied. In neither
+case is a reference to the message representation kept by the :class:`Mailbox`
+instance.
+
+The default :class:`Mailbox` iterator iterates over message representations, not
+keys as the default dictionary iterator does. Moreover, modification of a
+mailbox during iteration is safe and well-defined. Messages added to the mailbox
+after an iterator is created will not be seen by the iterator. Messages removed
+from the mailbox before the iterator yields them will be silently skipped,
+though using a key from an iterator may result in a :exc:`KeyError` exception if
+the corresponding message is subsequently removed.
+
+.. warning::
+
+   Be very cautious when modifying mailboxes that might be simultaneously changed
+   by some other process.  The safest mailbox format to use for such tasks is
+   Maildir; try to avoid using single-file formats such as mbox for concurrent
+   writing.  If you're modifying a mailbox, you *must* lock it by calling the
+   :meth:`lock` and :meth:`unlock` methods *before* reading any messages in the
+   file or making any changes by adding or deleting a message.  Failing to lock the
+   mailbox runs the risk of losing messages or corrupting the entire mailbox.
+
+:class:`Mailbox` instances have the following methods:
+
+
+.. method:: Mailbox.add(message)
+
+   Add *message* to the mailbox and return the key that has been assigned to it.
+
+   Parameter *message* may be a :class:`Message` instance, an
+   :class:`email.Message.Message` instance, a string, or a file-like object (which
+   should be open in text mode). If *message* is an instance of the appropriate
+   format-specific :class:`Message` subclass (e.g., if it's an :class:`mboxMessage`
+   instance and this is an :class:`mbox` instance), its format-specific information
+   is used. Otherwise, reasonable defaults for format-specific information are
+   used.
+
+
+.. method:: Mailbox.remove(key)
+            Mailbox.__delitem__(key)
+            Mailbox.discard(key)
+
+   Delete the message corresponding to *key* from the mailbox.
+
+   If no such message exists, a :exc:`KeyError` exception is raised if the method
+   was called as :meth:`remove` or :meth:`__delitem__` but no exception is raised
+   if the method was called as :meth:`discard`. The behavior of :meth:`discard` may
+   be preferred if the underlying mailbox format supports concurrent modification
+   by other processes.
+
+
+.. method:: Mailbox.__setitem__(key, message)
+
+   Replace the message corresponding to *key* with *message*. Raise a
+   :exc:`KeyError` exception if no message already corresponds to *key*.
+
+   As with :meth:`add`, parameter *message* may be a :class:`Message` instance, an
+   :class:`email.Message.Message` instance, a string, or a file-like object (which
+   should be open in text mode). If *message* is an instance of the appropriate
+   format-specific :class:`Message` subclass (e.g., if it's an :class:`mboxMessage`
+   instance and this is an :class:`mbox` instance), its format-specific information
+   is used. Otherwise, the format-specific information of the message that
+   currently corresponds to *key* is left unchanged.
+
+
+.. method:: Mailbox.iterkeys()
+            Mailbox.keys()
+
+   Return an iterator over all keys if called as :meth:`iterkeys` or return a list
+   of keys if called as :meth:`keys`.
+
+
+.. method:: Mailbox.itervalues()
+            Mailbox.__iter__()
+            Mailbox.values()
+
+   Return an iterator over representations of all messages if called as
+   :meth:`itervalues` or :meth:`__iter__` or return a list of such representations
+   if called as :meth:`values`. The messages are represented as instances of the
+   appropriate format-specific :class:`Message` subclass unless a custom message
+   factory was specified when the :class:`Mailbox` instance was initialized.
+
+   .. note::
+
+      The behavior of :meth:`__iter__` is unlike that of dictionaries, which iterate
+      over keys.
+
+
+.. method:: Mailbox.iteritems()
+            Mailbox.items()
+
+   Return an iterator over (*key*, *message*) pairs, where *key* is a key and
+   *message* is a message representation, if called as :meth:`iteritems` or return
+   a list of such pairs if called as :meth:`items`. The messages are represented as
+   instances of the appropriate format-specific :class:`Message` subclass unless a
+   custom message factory was specified when the :class:`Mailbox` instance was
+   initialized.
+
+
+.. method:: Mailbox.get(key[, default=None])
+            Mailbox.__getitem__(key)
+
+   Return a representation of the message corresponding to *key*. If no such
+   message exists, *default* is returned if the method was called as :meth:`get`
+   and a :exc:`KeyError` exception is raised if the method was called as
+   :meth:`__getitem__`. The message is represented as an instance of the
+   appropriate format-specific :class:`Message` subclass unless a custom message
+   factory was specified when the :class:`Mailbox` instance was initialized.
+
+
+.. method:: Mailbox.get_message(key)
+
+   Return a representation of the message corresponding to *key* as an instance of
+   the appropriate format-specific :class:`Message` subclass, or raise a
+   :exc:`KeyError` exception if no such message exists.
+
+
+.. method:: Mailbox.get_string(key)
+
+   Return a string representation of the message corresponding to *key*, or raise a
+   :exc:`KeyError` exception if no such message exists.
+
+
+.. method:: Mailbox.get_file(key)
+
+   Return a file-like representation of the message corresponding to *key*, or
+   raise a :exc:`KeyError` exception if no such message exists. The file-like
+   object behaves as if open in binary mode. This file should be closed once it is
+   no longer needed.
+
+   .. note::
+
+      Unlike other representations of messages, file-like representations are not
+      necessarily independent of the :class:`Mailbox` instance that created them or of
+      the underlying mailbox. More specific documentation is provided by each
+      subclass.
+
+
+.. method:: Mailbox.has_key(key)
+            Mailbox.__contains__(key)
+
+   Return ``True`` if *key* corresponds to a message, ``False`` otherwise.
+
+
+.. method:: Mailbox.__len__()
+
+   Return a count of messages in the mailbox.
+
+
+.. method:: Mailbox.clear()
+
+   Delete all messages from the mailbox.
+
+
+.. method:: Mailbox.pop(key[, default])
+
+   Return a representation of the message corresponding to *key* and delete the
+   message. If no such message exists, return *default* if it was supplied or else
+   raise a :exc:`KeyError` exception. The message is represented as an instance of
+   the appropriate format-specific :class:`Message` subclass unless a custom
+   message factory was specified when the :class:`Mailbox` instance was
+   initialized.
+
+
+.. method:: Mailbox.popitem()
+
+   Return an arbitrary (*key*, *message*) pair, where *key* is a key and *message*
+   is a message representation, and delete the corresponding message. If the
+   mailbox is empty, raise a :exc:`KeyError` exception. The message is represented
+   as an instance of the appropriate format-specific :class:`Message` subclass
+   unless a custom message factory was specified when the :class:`Mailbox` instance
+   was initialized.
+
+
+.. method:: Mailbox.update(arg)
+
+   Parameter *arg* should be a *key*-to-*message* mapping or an iterable of (*key*,
+   *message*) pairs. Updates the mailbox so that, for each given *key* and
+   *message*, the message corresponding to *key* is set to *message* as if by using
+   :meth:`__setitem__`. As with :meth:`__setitem__`, each *key* must already
+   correspond to a message in the mailbox or else a :exc:`KeyError` exception will
+   be raised, so in general it is incorrect for *arg* to be a :class:`Mailbox`
+   instance.
+
+   .. note::
+
+      Unlike with dictionaries, keyword arguments are not supported.
+
+
+.. method:: Mailbox.flush()
+
+   Write any pending changes to the filesystem. For some :class:`Mailbox`
+   subclasses, changes are always written immediately and :meth:`flush` does
+   nothing, but you should still make a habit of calling this method.
+
+
+.. method:: Mailbox.lock()
+
+   Acquire an exclusive advisory lock on the mailbox so that other processes know
+   not to modify it. An :exc:`ExternalClashError` is raised if the lock is not
+   available. The particular locking mechanisms used depend upon the mailbox
+   format.  You should *always* lock the mailbox before making any  modifications
+   to its contents.
+
+
+.. method:: Mailbox.unlock()
+
+   Release the lock on the mailbox, if any.
+
+
+.. method:: Mailbox.close()
+
+   Flush the mailbox, unlock it if necessary, and close any open files. For some
+   :class:`Mailbox` subclasses, this method does nothing.
+
+
+.. _mailbox-maildir:
+
+:class:`Maildir`
+^^^^^^^^^^^^^^^^
+
+
+.. class:: Maildir(dirname[, factory=rfc822.Message[, create=True]])
+
+   A subclass of :class:`Mailbox` for mailboxes in Maildir format. Parameter
+   *factory* is a callable object that accepts a file-like message representation
+   (which behaves as if opened in binary mode) and returns a custom representation.
+   If *factory* is ``None``, :class:`MaildirMessage` is used as the default message
+   representation. If *create* is ``True``, the mailbox is created if it does not
+   exist.
+
+   It is for historical reasons that *factory* defaults to :class:`rfc822.Message`
+   and that *dirname* is named as such rather than *path*. For a :class:`Maildir`
+   instance that behaves like instances of other :class:`Mailbox` subclasses, set
+   *factory* to ``None``.
+
+Maildir is a directory-based mailbox format invented for the qmail mail transfer
+agent and now widely supported by other programs. Messages in a Maildir mailbox
+are stored in separate files within a common directory structure. This design
+allows Maildir mailboxes to be accessed and modified by multiple unrelated
+programs without data corruption, so file locking is unnecessary.
+
+Maildir mailboxes contain three subdirectories, namely: :file:`tmp`,
+:file:`new`, and :file:`cur`. Messages are created momentarily in the
+:file:`tmp` subdirectory and then moved to the :file:`new` subdirectory to
+finalize delivery. A mail user agent may subsequently move the message to the
+:file:`cur` subdirectory and store information about the state of the message in
+a special "info" section appended to its file name.
+
+Folders of the style introduced by the Courier mail transfer agent are also
+supported. Any subdirectory of the main mailbox is considered a folder if
+``'.'`` is the first character in its name. Folder names are represented by
+:class:`Maildir` without the leading ``'.'``. Each folder is itself a Maildir
+mailbox but should not contain other folders. Instead, a logical nesting is
+indicated using ``'.'`` to delimit levels, e.g., "Archived.2005.07".
+
+.. note::
+
+   The Maildir specification requires the use of a colon (``':'``) in certain
+   message file names. However, some operating systems do not permit this character
+   in file names, If you wish to use a Maildir-like format on such an operating
+   system, you should specify another character to use instead. The exclamation
+   point (``'!'``) is a popular choice. For example::
+
+      import mailbox
+      mailbox.Maildir.colon = '!'
+
+   The :attr:`colon` attribute may also be set on a per-instance basis.
+
+:class:`Maildir` instances have all of the methods of :class:`Mailbox` in
+addition to the following:
+
+
+.. method:: Maildir.list_folders()
+
+   Return a list of the names of all folders.
+
+
+.. method:: Maildir.get_folder(folder)
+
+   Return a :class:`Maildir` instance representing the folder whose name is
+   *folder*. A :exc:`NoSuchMailboxError` exception is raised if the folder does not
+   exist.
+
+
+.. method:: Maildir.add_folder(folder)
+
+   Create a folder whose name is *folder* and return a :class:`Maildir` instance
+   representing it.
+
+
+.. method:: Maildir.remove_folder(folder)
+
+   Delete the folder whose name is *folder*. If the folder contains any messages, a
+   :exc:`NotEmptyError` exception will be raised and the folder will not be
+   deleted.
+
+
+.. method:: Maildir.clean()
+
+   Delete temporary files from the mailbox that have not been accessed in the last
+   36 hours. The Maildir specification says that mail-reading programs should do
+   this occasionally.
+
+Some :class:`Mailbox` methods implemented by :class:`Maildir` deserve special
+remarks:
+
+
+.. method:: Maildir.add(message)
+            Maildir.__setitem__(key, message)
+            Maildir.update(arg)
+
+   .. warning::
+
+      These methods generate unique file names based upon the current process ID. When
+      using multiple threads, undetected name clashes may occur and cause corruption
+      of the mailbox unless threads are coordinated to avoid using these methods to
+      manipulate the same mailbox simultaneously.
+
+
+.. method:: Maildir.flush()
+
+   All changes to Maildir mailboxes are immediately applied, so this method does
+   nothing.
+
+
+.. method:: Maildir.lock()
+            Maildir.unlock()
+
+   Maildir mailboxes do not support (or require) locking, so these methods do
+   nothing.
+
+
+.. method:: Maildir.close()
+
+   :class:`Maildir` instances do not keep any open files and the underlying
+   mailboxes do not support locking, so this method does nothing.
+
+
+.. method:: Maildir.get_file(key)
+
+   Depending upon the host platform, it may not be possible to modify or remove the
+   underlying message while the returned file remains open.
+
+
+.. seealso::
+
+   `maildir man page from qmail <http://www.qmail.org/man/man5/maildir.html>`_
+      The original specification of the format.
+
+   `Using maildir format <http://cr.yp.to/proto/maildir.html>`_
+      Notes on Maildir by its inventor. Includes an updated name-creation scheme and
+      details on "info" semantics.
+
+   `maildir man page from Courier <http://www.courier-mta.org/?maildir.html>`_
+      Another specification of the format. Describes a common extension for supporting
+      folders.
+
+
+.. _mailbox-mbox:
+
+:class:`mbox`
+^^^^^^^^^^^^^
+
+
+.. class:: mbox(path[, factory=None[, create=True]])
+
+   A subclass of :class:`Mailbox` for mailboxes in mbox format. Parameter *factory*
+   is a callable object that accepts a file-like message representation (which
+   behaves as if opened in binary mode) and returns a custom representation. If
+   *factory* is ``None``, :class:`mboxMessage` is used as the default message
+   representation. If *create* is ``True``, the mailbox is created if it does not
+   exist.
+
+The mbox format is the classic format for storing mail on Unix systems. All
+messages in an mbox mailbox are stored in a single file with the beginning of
+each message indicated by a line whose first five characters are "From ".
+
+Several variations of the mbox format exist to address perceived shortcomings in
+the original. In the interest of compatibility, :class:`mbox` implements the
+original format, which is sometimes referred to as :dfn:`mboxo`. This means that
+the :mailheader:`Content-Length` header, if present, is ignored and that any
+occurrences of "From " at the beginning of a line in a message body are
+transformed to ">From " when storing the message, although occurences of ">From
+" are not transformed to "From " when reading the message.
+
+Some :class:`Mailbox` methods implemented by :class:`mbox` deserve special
+remarks:
+
+
+.. method:: mbox.get_file(key)
+
+   Using the file after calling :meth:`flush` or :meth:`close` on the :class:`mbox`
+   instance may yield unpredictable results or raise an exception.
+
+
+.. method:: mbox.lock()
+            mbox.unlock()
+
+   Three locking mechanisms are used---dot locking and, if available, the
+   :cfunc:`flock` and :cfunc:`lockf` system calls.
+
+
+.. seealso::
+
+   `mbox man page from qmail <http://www.qmail.org/man/man5/mbox.html>`_
+      A specification of the format and its variations.
+
+   `mbox man page from tin <http://www.tin.org/bin/man.cgi?section=5&topic=mbox>`_
+      Another specification of the format, with details on locking.
+
+   `Configuring Netscape Mail on Unix: Why The Content-Length Format is Bad <http://home.netscape.com/eng/mozilla/2.0/relnotes/demo/content-length.html>`_
+      An argument for using the original mbox format rather than a variation.
+
+   `"mbox" is a family of several mutually incompatible mailbox formats <http://homepages.tesco.net./~J.deBoynePollard/FGA/mail-mbox-formats.html>`_
+      A history of mbox variations.
+
+
+.. _mailbox-mh:
+
+:class:`MH`
+^^^^^^^^^^^
+
+
+.. class:: MH(path[, factory=None[, create=True]])
+
+   A subclass of :class:`Mailbox` for mailboxes in MH format. Parameter *factory*
+   is a callable object that accepts a file-like message representation (which
+   behaves as if opened in binary mode) and returns a custom representation. If
+   *factory* is ``None``, :class:`MHMessage` is used as the default message
+   representation. If *create* is ``True``, the mailbox is created if it does not
+   exist.
+
+MH is a directory-based mailbox format invented for the MH Message Handling
+System, a mail user agent. Each message in an MH mailbox resides in its own
+file. An MH mailbox may contain other MH mailboxes (called :dfn:`folders`) in
+addition to messages. Folders may be nested indefinitely. MH mailboxes also
+support :dfn:`sequences`, which are named lists used to logically group messages
+without moving them to sub-folders. Sequences are defined in a file called
+:file:`.mh_sequences` in each folder.
+
+The :class:`MH` class manipulates MH mailboxes, but it does not attempt to
+emulate all of :program:`mh`'s behaviors. In particular, it does not modify and
+is not affected by the :file:`context` or :file:`.mh_profile` files that are
+used by :program:`mh` to store its state and configuration.
+
+:class:`MH` instances have all of the methods of :class:`Mailbox` in addition to
+the following:
+
+
+.. method:: MH.list_folders()
+
+   Return a list of the names of all folders.
+
+
+.. method:: MH.get_folder(folder)
+
+   Return an :class:`MH` instance representing the folder whose name is *folder*. A
+   :exc:`NoSuchMailboxError` exception is raised if the folder does not exist.
+
+
+.. method:: MH.add_folder(folder)
+
+   Create a folder whose name is *folder* and return an :class:`MH` instance
+   representing it.
+
+
+.. method:: MH.remove_folder(folder)
+
+   Delete the folder whose name is *folder*. If the folder contains any messages, a
+   :exc:`NotEmptyError` exception will be raised and the folder will not be
+   deleted.
+
+
+.. method:: MH.get_sequences()
+
+   Return a dictionary of sequence names mapped to key lists. If there are no
+   sequences, the empty dictionary is returned.
+
+
+.. method:: MH.set_sequences(sequences)
+
+   Re-define the sequences that exist in the mailbox based upon *sequences*, a
+   dictionary of names mapped to key lists, like returned by :meth:`get_sequences`.
+
+
+.. method:: MH.pack()
+
+   Rename messages in the mailbox as necessary to eliminate gaps in numbering.
+   Entries in the sequences list are updated correspondingly.
+
+   .. note::
+
+      Already-issued keys are invalidated by this operation and should not be
+      subsequently used.
+
+Some :class:`Mailbox` methods implemented by :class:`MH` deserve special
+remarks:
+
+
+.. method:: MH.remove(key)
+            MH.__delitem__(key)
+            MH.discard(key)
+
+   These methods immediately delete the message. The MH convention of marking a
+   message for deletion by prepending a comma to its name is not used.
+
+
+.. method:: MH.lock()
+            MH.unlock()
+
+   Three locking mechanisms are used---dot locking and, if available, the
+   :cfunc:`flock` and :cfunc:`lockf` system calls. For MH mailboxes, locking the
+   mailbox means locking the :file:`.mh_sequences` file and, only for the duration
+   of any operations that affect them, locking individual message files.
+
+
+.. method:: MH.get_file(key)
+
+   Depending upon the host platform, it may not be possible to remove the
+   underlying message while the returned file remains open.
+
+
+.. method:: MH.flush()
+
+   All changes to MH mailboxes are immediately applied, so this method does
+   nothing.
+
+
+.. method:: MH.close()
+
+   :class:`MH` instances do not keep any open files, so this method is equivelant
+   to :meth:`unlock`.
+
+
+.. seealso::
+
+   `nmh - Message Handling System <http://www.nongnu.org/nmh/>`_
+      Home page of :program:`nmh`, an updated version of the original :program:`mh`.
+
+   `MH & nmh: Email for Users & Programmers <http://www.ics.uci.edu/~mh/book/>`_
+      A GPL-licensed book on :program:`mh` and :program:`nmh`, with some information
+      on the mailbox format.
+
+
+.. _mailbox-babyl:
+
+:class:`Babyl`
+^^^^^^^^^^^^^^
+
+
+.. class:: Babyl(path[, factory=None[, create=True]])
+
+   A subclass of :class:`Mailbox` for mailboxes in Babyl format. Parameter
+   *factory* is a callable object that accepts a file-like message representation
+   (which behaves as if opened in binary mode) and returns a custom representation.
+   If *factory* is ``None``, :class:`BabylMessage` is used as the default message
+   representation. If *create* is ``True``, the mailbox is created if it does not
+   exist.
+
+Babyl is a single-file mailbox format used by the Rmail mail user agent included
+with Emacs. The beginning of a message is indicated by a line containing the two
+characters Control-Underscore (``'\037'``) and Control-L (``'\014'``). The end
+of a message is indicated by the start of the next message or, in the case of
+the last message, a line containing a Control-Underscore (``'\037'``)
+character.
+
+Messages in a Babyl mailbox have two sets of headers, original headers and
+so-called visible headers. Visible headers are typically a subset of the
+original headers that have been reformatted or abridged to be more
+attractive. Each message in a Babyl mailbox also has an accompanying list of
+:dfn:`labels`, or short strings that record extra information about the message,
+and a list of all user-defined labels found in the mailbox is kept in the Babyl
+options section.
+
+:class:`Babyl` instances have all of the methods of :class:`Mailbox` in addition
+to the following:
+
+
+.. method:: Babyl.get_labels()
+
+   Return a list of the names of all user-defined labels used in the mailbox.
+
+   .. note::
+
+      The actual messages are inspected to determine which labels exist in the mailbox
+      rather than consulting the list of labels in the Babyl options section, but the
+      Babyl section is updated whenever the mailbox is modified.
+
+Some :class:`Mailbox` methods implemented by :class:`Babyl` deserve special
+remarks:
+
+
+.. method:: Babyl.get_file(key)
+
+   In Babyl mailboxes, the headers of a message are not stored contiguously with
+   the body of the message. To generate a file-like representation, the headers and
+   body are copied together into a :class:`StringIO` instance (from the
+   :mod:`StringIO` module), which has an API identical to that of a file. As a
+   result, the file-like object is truly independent of the underlying mailbox but
+   does not save memory compared to a string representation.
+
+
+.. method:: Babyl.lock()
+            Babyl.unlock()
+
+   Three locking mechanisms are used---dot locking and, if available, the
+   :cfunc:`flock` and :cfunc:`lockf` system calls.
+
+
+.. seealso::
+
+   `Format of Version 5 Babyl Files <http://quimby.gnus.org/notes/BABYL>`_
+      A specification of the Babyl format.
+
+   `Reading Mail with Rmail <http://www.gnu.org/software/emacs/manual/html_node/Rmail.html>`_
+      The Rmail manual, with some information on Babyl semantics.
+
+
+.. _mailbox-mmdf:
+
+:class:`MMDF`
+^^^^^^^^^^^^^
+
+
+.. class:: MMDF(path[, factory=None[, create=True]])
+
+   A subclass of :class:`Mailbox` for mailboxes in MMDF format. Parameter *factory*
+   is a callable object that accepts a file-like message representation (which
+   behaves as if opened in binary mode) and returns a custom representation. If
+   *factory* is ``None``, :class:`MMDFMessage` is used as the default message
+   representation. If *create* is ``True``, the mailbox is created if it does not
+   exist.
+
+MMDF is a single-file mailbox format invented for the Multichannel Memorandum
+Distribution Facility, a mail transfer agent. Each message is in the same form
+as an mbox message but is bracketed before and after by lines containing four
+Control-A (``'\001'``) characters. As with the mbox format, the beginning of
+each message is indicated by a line whose first five characters are "From ", but
+additional occurrences of "From " are not transformed to ">From " when storing
+messages because the extra message separator lines prevent mistaking such
+occurrences for the starts of subsequent messages.
+
+Some :class:`Mailbox` methods implemented by :class:`MMDF` deserve special
+remarks:
+
+
+.. method:: MMDF.get_file(key)
+
+   Using the file after calling :meth:`flush` or :meth:`close` on the :class:`MMDF`
+   instance may yield unpredictable results or raise an exception.
+
+
+.. method:: MMDF.lock()
+            MMDF.unlock()
+
+   Three locking mechanisms are used---dot locking and, if available, the
+   :cfunc:`flock` and :cfunc:`lockf` system calls.
+
+
+.. seealso::
+
+   `mmdf man page from tin <http://www.tin.org/bin/man.cgi?section=5&topic=mmdf>`_
+      A specification of MMDF format from the documentation of tin, a newsreader.
+
+   `MMDF <http://en.wikipedia.org/wiki/MMDF>`_
+      A Wikipedia article describing the Multichannel Memorandum Distribution
+      Facility.
+
+
+.. _mailbox-message-objects:
+
+:class:`Message` objects
+------------------------
+
+
+.. class:: Message([message])
+
+   A subclass of the :mod:`email.Message` module's :class:`Message`. Subclasses of
+   :class:`mailbox.Message` add mailbox-format-specific state and behavior.
+
+   If *message* is omitted, the new instance is created in a default, empty state.
+   If *message* is an :class:`email.Message.Message` instance, its contents are
+   copied; furthermore, any format-specific information is converted insofar as
+   possible if *message* is a :class:`Message` instance. If *message* is a string
+   or a file, it should contain an :rfc:`2822`\ -compliant message, which is read
+   and parsed.
+
+The format-specific state and behaviors offered by subclasses vary, but in
+general it is only the properties that are not specific to a particular mailbox
+that are supported (although presumably the properties are specific to a
+particular mailbox format). For example, file offsets for single-file mailbox
+formats and file names for directory-based mailbox formats are not retained,
+because they are only applicable to the original mailbox. But state such as
+whether a message has been read by the user or marked as important is retained,
+because it applies to the message itself.
+
+There is no requirement that :class:`Message` instances be used to represent
+messages retrieved using :class:`Mailbox` instances. In some situations, the
+time and memory required to generate :class:`Message` representations might not
+not acceptable. For such situations, :class:`Mailbox` instances also offer
+string and file-like representations, and a custom message factory may be
+specified when a :class:`Mailbox` instance is initialized.
+
+
+.. _mailbox-maildirmessage:
+
+:class:`MaildirMessage`
+^^^^^^^^^^^^^^^^^^^^^^^
+
+
+.. class:: MaildirMessage([message])
+
+   A message with Maildir-specific behaviors. Parameter *message* has the same
+   meaning as with the :class:`Message` constructor.
+
+Typically, a mail user agent application moves all of the messages in the
+:file:`new` subdirectory to the :file:`cur` subdirectory after the first time
+the user opens and closes the mailbox, recording that the messages are old
+whether or not they've actually been read. Each message in :file:`cur` has an
+"info" section added to its file name to store information about its state.
+(Some mail readers may also add an "info" section to messages in :file:`new`.)
+The "info" section may take one of two forms: it may contain "2," followed by a
+list of standardized flags (e.g., "2,FR") or it may contain "1," followed by
+so-called experimental information. Standard flags for Maildir messages are as
+follows:
+
++------+---------+--------------------------------+
+| Flag | Meaning | Explanation                    |
++======+=========+================================+
+| D    | Draft   | Under composition              |
++------+---------+--------------------------------+
+| F    | Flagged | Marked as important            |
++------+---------+--------------------------------+
+| P    | Passed  | Forwarded, resent, or bounced  |
++------+---------+--------------------------------+
+| R    | Replied | Replied to                     |
++------+---------+--------------------------------+
+| S    | Seen    | Read                           |
++------+---------+--------------------------------+
+| T    | Trashed | Marked for subsequent deletion |
++------+---------+--------------------------------+
+
+:class:`MaildirMessage` instances offer the following methods:
+
+
+.. method:: MaildirMessage.get_subdir()
+
+   Return either "new" (if the message should be stored in the :file:`new`
+   subdirectory) or "cur" (if the message should be stored in the :file:`cur`
+   subdirectory).
+
+   .. note::
+
+      A message is typically moved from :file:`new` to :file:`cur` after its mailbox
+      has been accessed, whether or not the message is has been read. A message
+      ``msg`` has been read if ``"S" not in msg.get_flags()`` is ``True``.
+
+
+.. method:: MaildirMessage.set_subdir(subdir)
+
+   Set the subdirectory the message should be stored in. Parameter *subdir* must be
+   either "new" or "cur".
+
+
+.. method:: MaildirMessage.get_flags()
+
+   Return a string specifying the flags that are currently set. If the message
+   complies with the standard Maildir format, the result is the concatenation in
+   alphabetical order of zero or one occurrence of each of ``'D'``, ``'F'``,
+   ``'P'``, ``'R'``, ``'S'``, and ``'T'``. The empty string is returned if no flags
+   are set or if "info" contains experimental semantics.
+
+
+.. method:: MaildirMessage.set_flags(flags)
+
+   Set the flags specified by *flags* and unset all others.
+
+
+.. method:: MaildirMessage.add_flag(flag)
+
+   Set the flag(s) specified by *flag* without changing other flags. To add more
+   than one flag at a time, *flag* may be a string of more than one character. The
+   current "info" is overwritten whether or not it contains experimental
+   information rather than flags.
+
+
+.. method:: MaildirMessage.remove_flag(flag)
+
+   Unset the flag(s) specified by *flag* without changing other flags. To remove
+   more than one flag at a time, *flag* maybe a string of more than one character.
+   If "info" contains experimental information rather than flags, the current
+   "info" is not modified.
+
+
+.. method:: MaildirMessage.get_date()
+
+   Return the delivery date of the message as a floating-point number representing
+   seconds since the epoch.
+
+
+.. method:: MaildirMessage.set_date(date)
+
+   Set the delivery date of the message to *date*, a floating-point number
+   representing seconds since the epoch.
+
+
+.. method:: MaildirMessage.get_info()
+
+   Return a string containing the "info" for a message. This is useful for
+   accessing and modifying "info" that is experimental (i.e., not a list of flags).
+
+
+.. method:: MaildirMessage.set_info(info)
+
+   Set "info" to *info*, which should be a string.
+
+When a :class:`MaildirMessage` instance is created based upon an
+:class:`mboxMessage` or :class:`MMDFMessage` instance, the :mailheader:`Status`
+and :mailheader:`X-Status` headers are omitted and the following conversions
+take place:
+
++--------------------+----------------------------------------------+
+| Resulting state    | :class:`mboxMessage` or :class:`MMDFMessage` |
+|                    | state                                        |
++====================+==============================================+
+| "cur" subdirectory | O flag                                       |
++--------------------+----------------------------------------------+
+| F flag             | F flag                                       |
++--------------------+----------------------------------------------+
+| R flag             | A flag                                       |
++--------------------+----------------------------------------------+
+| S flag             | R flag                                       |
++--------------------+----------------------------------------------+
+| T flag             | D flag                                       |
++--------------------+----------------------------------------------+
+
+When a :class:`MaildirMessage` instance is created based upon an
+:class:`MHMessage` instance, the following conversions take place:
+
++-------------------------------+--------------------------+
+| Resulting state               | :class:`MHMessage` state |
++===============================+==========================+
+| "cur" subdirectory            | "unseen" sequence        |
++-------------------------------+--------------------------+
+| "cur" subdirectory and S flag | no "unseen" sequence     |
++-------------------------------+--------------------------+
+| F flag                        | "flagged" sequence       |
++-------------------------------+--------------------------+
+| R flag                        | "replied" sequence       |
++-------------------------------+--------------------------+
+
+When a :class:`MaildirMessage` instance is created based upon a
+:class:`BabylMessage` instance, the following conversions take place:
+
++-------------------------------+-------------------------------+
+| Resulting state               | :class:`BabylMessage` state   |
++===============================+===============================+
+| "cur" subdirectory            | "unseen" label                |
++-------------------------------+-------------------------------+
+| "cur" subdirectory and S flag | no "unseen" label             |
++-------------------------------+-------------------------------+
+| P flag                        | "forwarded" or "resent" label |
++-------------------------------+-------------------------------+
+| R flag                        | "answered" label              |
++-------------------------------+-------------------------------+
+| T flag                        | "deleted" label               |
++-------------------------------+-------------------------------+
+
+
+.. _mailbox-mboxmessage:
+
+:class:`mboxMessage`
+^^^^^^^^^^^^^^^^^^^^
+
+
+.. class:: mboxMessage([message])
+
+   A message with mbox-specific behaviors. Parameter *message* has the same meaning
+   as with the :class:`Message` constructor.
+
+Messages in an mbox mailbox are stored together in a single file. The sender's
+envelope address and the time of delivery are typically stored in a line
+beginning with "From " that is used to indicate the start of a message, though
+there is considerable variation in the exact format of this data among mbox
+implementations. Flags that indicate the state of the message, such as whether
+it has been read or marked as important, are typically stored in
+:mailheader:`Status` and :mailheader:`X-Status` headers.
+
+Conventional flags for mbox messages are as follows:
+
++------+----------+--------------------------------+
+| Flag | Meaning  | Explanation                    |
++======+==========+================================+
+| R    | Read     | Read                           |
++------+----------+--------------------------------+
+| O    | Old      | Previously detected by MUA     |
++------+----------+--------------------------------+
+| D    | Deleted  | Marked for subsequent deletion |
++------+----------+--------------------------------+
+| F    | Flagged  | Marked as important            |
++------+----------+--------------------------------+
+| A    | Answered | Replied to                     |
++------+----------+--------------------------------+
+
+The "R" and "O" flags are stored in the :mailheader:`Status` header, and the
+"D", "F", and "A" flags are stored in the :mailheader:`X-Status` header. The
+flags and headers typically appear in the order mentioned.
+
+:class:`mboxMessage` instances offer the following methods:
+
+
+.. method:: mboxMessage.get_from()
+
+   Return a string representing the "From " line that marks the start of the
+   message in an mbox mailbox. The leading "From " and the trailing newline are
+   excluded.
+
+
+.. method:: mboxMessage.set_from(from_[, time_=None])
+
+   Set the "From " line to *from_*, which should be specified without a leading
+   "From " or trailing newline. For convenience, *time_* may be specified and will
+   be formatted appropriately and appended to *from_*. If *time_* is specified, it
+   should be a :class:`struct_time` instance, a tuple suitable for passing to
+   :meth:`time.strftime`, or ``True`` (to use :meth:`time.gmtime`).
+
+
+.. method:: mboxMessage.get_flags()
+
+   Return a string specifying the flags that are currently set. If the message
+   complies with the conventional format, the result is the concatenation in the
+   following order of zero or one occurrence of each of ``'R'``, ``'O'``, ``'D'``,
+   ``'F'``, and ``'A'``.
+
+
+.. method:: mboxMessage.set_flags(flags)
+
+   Set the flags specified by *flags* and unset all others. Parameter *flags*
+   should be the concatenation in any order of zero or more occurrences of each of
+   ``'R'``, ``'O'``, ``'D'``, ``'F'``, and ``'A'``.
+
+
+.. method:: mboxMessage.add_flag(flag)
+
+   Set the flag(s) specified by *flag* without changing other flags. To add more
+   than one flag at a time, *flag* may be a string of more than one character.
+
+
+.. method:: mboxMessage.remove_flag(flag)
+
+   Unset the flag(s) specified by *flag* without changing other flags. To remove
+   more than one flag at a time, *flag* maybe a string of more than one character.
+
+When an :class:`mboxMessage` instance is created based upon a
+:class:`MaildirMessage` instance, a "From " line is generated based upon the
+:class:`MaildirMessage` instance's delivery date, and the following conversions
+take place:
+
++-----------------+-------------------------------+
+| Resulting state | :class:`MaildirMessage` state |
++=================+===============================+
+| R flag          | S flag                        |
++-----------------+-------------------------------+
+| O flag          | "cur" subdirectory            |
++-----------------+-------------------------------+
+| D flag          | T flag                        |
++-----------------+-------------------------------+
+| F flag          | F flag                        |
++-----------------+-------------------------------+
+| A flag          | R flag                        |
++-----------------+-------------------------------+
+
+When an :class:`mboxMessage` instance is created based upon an
+:class:`MHMessage` instance, the following conversions take place:
+
++-------------------+--------------------------+
+| Resulting state   | :class:`MHMessage` state |
++===================+==========================+
+| R flag and O flag | no "unseen" sequence     |
++-------------------+--------------------------+
+| O flag            | "unseen" sequence        |
++-------------------+--------------------------+
+| F flag            | "flagged" sequence       |
++-------------------+--------------------------+
+| A flag            | "replied" sequence       |
++-------------------+--------------------------+
+
+When an :class:`mboxMessage` instance is created based upon a
+:class:`BabylMessage` instance, the following conversions take place:
+
++-------------------+-----------------------------+
+| Resulting state   | :class:`BabylMessage` state |
++===================+=============================+
+| R flag and O flag | no "unseen" label           |
++-------------------+-----------------------------+
+| O flag            | "unseen" label              |
++-------------------+-----------------------------+
+| D flag            | "deleted" label             |
++-------------------+-----------------------------+
+| A flag            | "answered" label            |
++-------------------+-----------------------------+
+
+When a :class:`Message` instance is created based upon an :class:`MMDFMessage`
+instance, the "From " line is copied and all flags directly correspond:
+
++-----------------+----------------------------+
+| Resulting state | :class:`MMDFMessage` state |
++=================+============================+
+| R flag          | R flag                     |
++-----------------+----------------------------+
+| O flag          | O flag                     |
++-----------------+----------------------------+
+| D flag          | D flag                     |
++-----------------+----------------------------+
+| F flag          | F flag                     |
++-----------------+----------------------------+
+| A flag          | A flag                     |
++-----------------+----------------------------+
+
+
+.. _mailbox-mhmessage:
+
+:class:`MHMessage`
+^^^^^^^^^^^^^^^^^^
+
+
+.. class:: MHMessage([message])
+
+   A message with MH-specific behaviors. Parameter *message* has the same meaning
+   as with the :class:`Message` constructor.
+
+MH messages do not support marks or flags in the traditional sense, but they do
+support sequences, which are logical groupings of arbitrary messages. Some mail
+reading programs (although not the standard :program:`mh` and :program:`nmh`)
+use sequences in much the same way flags are used with other formats, as
+follows:
+
++----------+------------------------------------------+
+| Sequence | Explanation                              |
++==========+==========================================+
+| unseen   | Not read, but previously detected by MUA |
++----------+------------------------------------------+
+| replied  | Replied to                               |
++----------+------------------------------------------+
+| flagged  | Marked as important                      |
++----------+------------------------------------------+
+
+:class:`MHMessage` instances offer the following methods:
+
+
+.. method:: MHMessage.get_sequences()
+
+   Return a list of the names of sequences that include this message.
+
+
+.. method:: MHMessage.set_sequences(sequences)
+
+   Set the list of sequences that include this message.
+
+
+.. method:: MHMessage.add_sequence(sequence)
+
+   Add *sequence* to the list of sequences that include this message.
+
+
+.. method:: MHMessage.remove_sequence(sequence)
+
+   Remove *sequence* from the list of sequences that include this message.
+
+When an :class:`MHMessage` instance is created based upon a
+:class:`MaildirMessage` instance, the following conversions take place:
+
++--------------------+-------------------------------+
+| Resulting state    | :class:`MaildirMessage` state |
++====================+===============================+
+| "unseen" sequence  | no S flag                     |
++--------------------+-------------------------------+
+| "replied" sequence | R flag                        |
++--------------------+-------------------------------+
+| "flagged" sequence | F flag                        |
++--------------------+-------------------------------+
+
+When an :class:`MHMessage` instance is created based upon an
+:class:`mboxMessage` or :class:`MMDFMessage` instance, the :mailheader:`Status`
+and :mailheader:`X-Status` headers are omitted and the following conversions
+take place:
+
++--------------------+----------------------------------------------+
+| Resulting state    | :class:`mboxMessage` or :class:`MMDFMessage` |
+|                    | state                                        |
++====================+==============================================+
+| "unseen" sequence  | no R flag                                    |
++--------------------+----------------------------------------------+
+| "replied" sequence | A flag                                       |
++--------------------+----------------------------------------------+
+| "flagged" sequence | F flag                                       |
++--------------------+----------------------------------------------+
+
+When an :class:`MHMessage` instance is created based upon a
+:class:`BabylMessage` instance, the following conversions take place:
+
++--------------------+-----------------------------+
+| Resulting state    | :class:`BabylMessage` state |
++====================+=============================+
+| "unseen" sequence  | "unseen" label              |
++--------------------+-----------------------------+
+| "replied" sequence | "answered" label            |
++--------------------+-----------------------------+
+
+
+.. _mailbox-babylmessage:
+
+:class:`BabylMessage`
+^^^^^^^^^^^^^^^^^^^^^
+
+
+.. class:: BabylMessage([message])
+
+   A message with Babyl-specific behaviors. Parameter *message* has the same
+   meaning as with the :class:`Message` constructor.
+
+Certain message labels, called :dfn:`attributes`, are defined by convention to
+have special meanings. The attributes are as follows:
+
++-----------+------------------------------------------+
+| Label     | Explanation                              |
++===========+==========================================+
+| unseen    | Not read, but previously detected by MUA |
++-----------+------------------------------------------+
+| deleted   | Marked for subsequent deletion           |
++-----------+------------------------------------------+
+| filed     | Copied to another file or mailbox        |
++-----------+------------------------------------------+
+| answered  | Replied to                               |
++-----------+------------------------------------------+
+| forwarded | Forwarded                                |
++-----------+------------------------------------------+
+| edited    | Modified by the user                     |
++-----------+------------------------------------------+
+| resent    | Resent                                   |
++-----------+------------------------------------------+
+
+By default, Rmail displays only visible headers. The :class:`BabylMessage`
+class, though, uses the original headers because they are more complete. Visible
+headers may be accessed explicitly if desired.
+
+:class:`BabylMessage` instances offer the following methods:
+
+
+.. method:: BabylMessage.get_labels()
+
+   Return a list of labels on the message.
+
+
+.. method:: BabylMessage.set_labels(labels)
+
+   Set the list of labels on the message to *labels*.
+
+
+.. method:: BabylMessage.add_label(label)
+
+   Add *label* to the list of labels on the message.
+
+
+.. method:: BabylMessage.remove_label(label)
+
+   Remove *label* from the list of labels on the message.
+
+
+.. method:: BabylMessage.get_visible()
+
+   Return an :class:`Message` instance whose headers are the message's visible
+   headers and whose body is empty.
+
+
+.. method:: BabylMessage.set_visible(visible)
+
+   Set the message's visible headers to be the same as the headers in *message*.
+   Parameter *visible* should be a :class:`Message` instance, an
+   :class:`email.Message.Message` instance, a string, or a file-like object (which
+   should be open in text mode).
+
+
+.. method:: BabylMessage.update_visible()
+
+   When a :class:`BabylMessage` instance's original headers are modified, the
+   visible headers are not automatically modified to correspond. This method
+   updates the visible headers as follows: each visible header with a corresponding
+   original header is set to the value of the original header, each visible header
+   without a corresponding original header is removed, and any of
+   :mailheader:`Date`, :mailheader:`From`, :mailheader:`Reply-To`,
+   :mailheader:`To`, :mailheader:`CC`, and :mailheader:`Subject` that are present
+   in the original headers but not the visible headers are added to the visible
+   headers.
+
+When a :class:`BabylMessage` instance is created based upon a
+:class:`MaildirMessage` instance, the following conversions take place:
+
++-------------------+-------------------------------+
+| Resulting state   | :class:`MaildirMessage` state |
++===================+===============================+
+| "unseen" label    | no S flag                     |
++-------------------+-------------------------------+
+| "deleted" label   | T flag                        |
++-------------------+-------------------------------+
+| "answered" label  | R flag                        |
++-------------------+-------------------------------+
+| "forwarded" label | P flag                        |
++-------------------+-------------------------------+
+
+When a :class:`BabylMessage` instance is created based upon an
+:class:`mboxMessage` or :class:`MMDFMessage` instance, the :mailheader:`Status`
+and :mailheader:`X-Status` headers are omitted and the following conversions
+take place:
+
++------------------+----------------------------------------------+
+| Resulting state  | :class:`mboxMessage` or :class:`MMDFMessage` |
+|                  | state                                        |
++==================+==============================================+
+| "unseen" label   | no R flag                                    |
++------------------+----------------------------------------------+
+| "deleted" label  | D flag                                       |
++------------------+----------------------------------------------+
+| "answered" label | A flag                                       |
++------------------+----------------------------------------------+
+
+When a :class:`BabylMessage` instance is created based upon an
+:class:`MHMessage` instance, the following conversions take place:
+
++------------------+--------------------------+
+| Resulting state  | :class:`MHMessage` state |
++==================+==========================+
+| "unseen" label   | "unseen" sequence        |
++------------------+--------------------------+
+| "answered" label | "replied" sequence       |
++------------------+--------------------------+
+
+
+.. _mailbox-mmdfmessage:
+
+:class:`MMDFMessage`
+^^^^^^^^^^^^^^^^^^^^
+
+
+.. class:: MMDFMessage([message])
+
+   A message with MMDF-specific behaviors. Parameter *message* has the same meaning
+   as with the :class:`Message` constructor.
+
+As with message in an mbox mailbox, MMDF messages are stored with the sender's
+address and the delivery date in an initial line beginning with "From ".
+Likewise, flags that indicate the state of the message are typically stored in
+:mailheader:`Status` and :mailheader:`X-Status` headers.
+
+Conventional flags for MMDF messages are identical to those of mbox message and
+are as follows:
+
++------+----------+--------------------------------+
+| Flag | Meaning  | Explanation                    |
++======+==========+================================+
+| R    | Read     | Read                           |
++------+----------+--------------------------------+
+| O    | Old      | Previously detected by MUA     |
++------+----------+--------------------------------+
+| D    | Deleted  | Marked for subsequent deletion |
++------+----------+--------------------------------+
+| F    | Flagged  | Marked as important            |
++------+----------+--------------------------------+
+| A    | Answered | Replied to                     |
++------+----------+--------------------------------+
+
+The "R" and "O" flags are stored in the :mailheader:`Status` header, and the
+"D", "F", and "A" flags are stored in the :mailheader:`X-Status` header. The
+flags and headers typically appear in the order mentioned.
+
+:class:`MMDFMessage` instances offer the following methods, which are identical
+to those offered by :class:`mboxMessage`:
+
+
+.. method:: MMDFMessage.get_from()
+
+   Return a string representing the "From " line that marks the start of the
+   message in an mbox mailbox. The leading "From " and the trailing newline are
+   excluded.
+
+
+.. method:: MMDFMessage.set_from(from_[, time_=None])
+
+   Set the "From " line to *from_*, which should be specified without a leading
+   "From " or trailing newline. For convenience, *time_* may be specified and will
+   be formatted appropriately and appended to *from_*. If *time_* is specified, it
+   should be a :class:`struct_time` instance, a tuple suitable for passing to
+   :meth:`time.strftime`, or ``True`` (to use :meth:`time.gmtime`).
+
+
+.. method:: MMDFMessage.get_flags()
+
+   Return a string specifying the flags that are currently set. If the message
+   complies with the conventional format, the result is the concatenation in the
+   following order of zero or one occurrence of each of ``'R'``, ``'O'``, ``'D'``,
+   ``'F'``, and ``'A'``.
+
+
+.. method:: MMDFMessage.set_flags(flags)
+
+   Set the flags specified by *flags* and unset all others. Parameter *flags*
+   should be the concatenation in any order of zero or more occurrences of each of
+   ``'R'``, ``'O'``, ``'D'``, ``'F'``, and ``'A'``.
+
+
+.. method:: MMDFMessage.add_flag(flag)
+
+   Set the flag(s) specified by *flag* without changing other flags. To add more
+   than one flag at a time, *flag* may be a string of more than one character.
+
+
+.. method:: MMDFMessage.remove_flag(flag)
+
+   Unset the flag(s) specified by *flag* without changing other flags. To remove
+   more than one flag at a time, *flag* maybe a string of more than one character.
+
+When an :class:`MMDFMessage` instance is created based upon a
+:class:`MaildirMessage` instance, a "From " line is generated based upon the
+:class:`MaildirMessage` instance's delivery date, and the following conversions
+take place:
+
++-----------------+-------------------------------+
+| Resulting state | :class:`MaildirMessage` state |
++=================+===============================+
+| R flag          | S flag                        |
++-----------------+-------------------------------+
+| O flag          | "cur" subdirectory            |
++-----------------+-------------------------------+
+| D flag          | T flag                        |
++-----------------+-------------------------------+
+| F flag          | F flag                        |
++-----------------+-------------------------------+
+| A flag          | R flag                        |
++-----------------+-------------------------------+
+
+When an :class:`MMDFMessage` instance is created based upon an
+:class:`MHMessage` instance, the following conversions take place:
+
++-------------------+--------------------------+
+| Resulting state   | :class:`MHMessage` state |
++===================+==========================+
+| R flag and O flag | no "unseen" sequence     |
++-------------------+--------------------------+
+| O flag            | "unseen" sequence        |
++-------------------+--------------------------+
+| F flag            | "flagged" sequence       |
++-------------------+--------------------------+
+| A flag            | "replied" sequence       |
++-------------------+--------------------------+
+
+When an :class:`MMDFMessage` instance is created based upon a
+:class:`BabylMessage` instance, the following conversions take place:
+
++-------------------+-----------------------------+
+| Resulting state   | :class:`BabylMessage` state |
++===================+=============================+
+| R flag and O flag | no "unseen" label           |
++-------------------+-----------------------------+
+| O flag            | "unseen" label              |
++-------------------+-----------------------------+
+| D flag            | "deleted" label             |
++-------------------+-----------------------------+
+| A flag            | "answered" label            |
++-------------------+-----------------------------+
+
+When an :class:`MMDFMessage` instance is created based upon an
+:class:`mboxMessage` instance, the "From " line is copied and all flags directly
+correspond:
+
++-----------------+----------------------------+
+| Resulting state | :class:`mboxMessage` state |
++=================+============================+
+| R flag          | R flag                     |
++-----------------+----------------------------+
+| O flag          | O flag                     |
++-----------------+----------------------------+
+| D flag          | D flag                     |
++-----------------+----------------------------+
+| F flag          | F flag                     |
++-----------------+----------------------------+
+| A flag          | A flag                     |
++-----------------+----------------------------+
+
+
+Exceptions
+----------
+
+The following exception classes are defined in the :mod:`mailbox` module:
+
+
+.. class:: Error()
+
+   The based class for all other module-specific exceptions.
+
+
+.. class:: NoSuchMailboxError()
+
+   Raised when a mailbox is expected but is not found, such as when instantiating a
+   :class:`Mailbox` subclass with a path that does not exist (and with the *create*
+   parameter set to ``False``), or when opening a folder that does not exist.
+
+
+.. class:: NotEmptyErrorError()
+
+   Raised when a mailbox is not empty but is expected to be, such as when deleting
+   a folder that contains messages.
+
+
+.. class:: ExternalClashError()
+
+   Raised when some mailbox-related condition beyond the control of the program
+   causes it to be unable to proceed, such as when failing to acquire a lock that
+   another program already holds a lock, or when a uniquely-generated file name
+   already exists.
+
+
+.. class:: FormatError()
+
+   Raised when the data in a file cannot be parsed, such as when an :class:`MH`
+   instance attempts to read a corrupted :file:`.mh_sequences` file.
+
+
+.. _mailbox-deprecated:
+
+Deprecated classes and methods
+------------------------------
+
+Older versions of the :mod:`mailbox` module do not support modification of
+mailboxes, such as adding or removing message, and do not provide classes to
+represent format-specific message properties. For backward compatibility, the
+older mailbox classes are still available, but the newer classes should be used
+in preference to them.
+
+Older mailbox objects support only iteration and provide a single public method:
+
+
+.. method:: oldmailbox.next()
+
+   Return the next message in the mailbox, created with the optional *factory*
+   argument passed into the mailbox object's constructor. By default this is an
+   :class:`rfc822.Message` object (see the :mod:`rfc822` module).  Depending on the
+   mailbox implementation the *fp* attribute of this object may be a true file
+   object or a class instance simulating a file object, taking care of things like
+   message boundaries if multiple mail messages are contained in a single file,
+   etc.  If no more messages are available, this method returns ``None``.
+
+Most of the older mailbox classes have names that differ from the current
+mailbox class names, except for :class:`Maildir`. For this reason, the new
+:class:`Maildir` class defines a :meth:`next` method and its constructor differs
+slightly from those of the other new mailbox classes.
+
+The older mailbox classes whose names are not the same as their newer
+counterparts are as follows:
+
+
+.. class:: UnixMailbox(fp[, factory])
+
+   Access to a classic Unix-style mailbox, where all messages are contained in a
+   single file and separated by ``From`` (a.k.a. ``From_``) lines.  The file object
+   *fp* points to the mailbox file.  The optional *factory* parameter is a callable
+   that should create new message objects.  *factory* is called with one argument,
+   *fp* by the :meth:`next` method of the mailbox object.  The default is the
+   :class:`rfc822.Message` class (see the :mod:`rfc822` module -- and the note
+   below).
+
+   .. note::
+
+      For reasons of this module's internal implementation, you will probably want to
+      open the *fp* object in binary mode.  This is especially important on Windows.
+
+   For maximum portability, messages in a Unix-style mailbox are separated by any
+   line that begins exactly with the string ``'From '`` (note the trailing space)
+   if preceded by exactly two newlines. Because of the wide-range of variations in
+   practice, nothing else on the ``From_`` line should be considered.  However, the
+   current implementation doesn't check for the leading two newlines.  This is
+   usually fine for most applications.
+
+   The :class:`UnixMailbox` class implements a more strict version of ``From_``
+   line checking, using a regular expression that usually correctly matched
+   ``From_`` delimiters.  It considers delimiter line to be separated by ``From
+   name time`` lines.  For maximum portability, use the
+   :class:`PortableUnixMailbox` class instead.  This class is identical to
+   :class:`UnixMailbox` except that individual messages are separated by only
+   ``From`` lines.
+
+   For more information, see `Configuring Netscape Mail on Unix: Why the
+   Content-Length Format is Bad
+   <http://home.netscape.com/eng/mozilla/2.0/relnotes/demo/content-length.html>`_.
+
+
+.. class:: PortableUnixMailbox(fp[, factory])
+
+   A less-strict version of :class:`UnixMailbox`, which considers only the ``From``
+   at the beginning of the line separating messages.  The "*name* *time*" portion
+   of the From line is ignored, to protect against some variations that are
+   observed in practice.  This works since lines in the message which begin with
+   ``'From '`` are quoted by mail handling software at delivery-time.
+
+
+.. class:: MmdfMailbox(fp[, factory])
+
+   Access an MMDF-style mailbox, where all messages are contained in a single file
+   and separated by lines consisting of 4 control-A characters.  The file object
+   *fp* points to the mailbox file. Optional *factory* is as with the
+   :class:`UnixMailbox` class.
+
+
+.. class:: MHMailbox(dirname[, factory])
+
+   Access an MH mailbox, a directory with each message in a separate file with a
+   numeric name. The name of the mailbox directory is passed in *dirname*.
+   *factory* is as with the :class:`UnixMailbox` class.
+
+
+.. class:: BabylMailbox(fp[, factory])
+
+   Access a Babyl mailbox, which is similar to an MMDF mailbox.  In Babyl format,
+   each message has two sets of headers, the *original* headers and the *visible*
+   headers.  The original headers appear before a line containing only ``'*** EOOH
+   ***'`` (End-Of-Original-Headers) and the visible headers appear after the
+   ``EOOH`` line.  Babyl-compliant mail readers will show you only the visible
+   headers, and :class:`BabylMailbox` objects will return messages containing only
+   the visible headers.  You'll have to do your own parsing of the mailbox file to
+   get at the original headers.  Mail messages start with the EOOH line and end
+   with a line containing only ``'\037\014'``.  *factory* is as with the
+   :class:`UnixMailbox` class.
+
+If you wish to use the older mailbox classes with the :mod:`email` module rather
+than the deprecated :mod:`rfc822` module, you can do so as follows::
+
+   import email
+   import email.Errors
+   import mailbox
+
+   def msgfactory(fp):
+       try:
+           return email.message_from_file(fp)
+       except email.Errors.MessageParseError:
+           # Don't return None since that will
+           # stop the mailbox iterator
+           return ''
+
+   mbox = mailbox.UnixMailbox(fp, msgfactory)
+
+Alternatively, if you know your mailbox contains only well-formed MIME messages,
+you can simplify this to::
+
+   import email
+   import mailbox
+
+   mbox = mailbox.UnixMailbox(fp, email.message_from_file)
+
+
+.. _mailbox-examples:
+
+Examples
+--------
+
+A simple example of printing the subjects of all messages in a mailbox that seem
+interesting::
+
+   import mailbox
+   for message in mailbox.mbox('~/mbox'):
+       subject = message['subject']       # Could possibly be None.
+       if subject and 'python' in subject.lower():
+           print subject
+
+To copy all mail from a Babyl mailbox to an MH mailbox, converting all of the
+format-specific information that can be converted::
+
+   import mailbox
+   destination = mailbox.MH('~/Mail')
+   destination.lock()
+   for message in mailbox.Babyl('~/RMAIL'):
+       destination.add(MHMessage(message))
+   destination.flush()
+   destination.unlock()
+
+This example sorts mail from several mailing lists into different mailboxes,
+being careful to avoid mail corruption due to concurrent modification by other
+programs, mail loss due to interruption of the program, or premature termination
+due to malformed messages in the mailbox::
+
+   import mailbox
+   import email.Errors
+
+   list_names = ('python-list', 'python-dev', 'python-bugs')
+
+   boxes = dict((name, mailbox.mbox('~/email/%s' % name)) for name in list_names)
+   inbox = mailbox.Maildir('~/Maildir', factory=None)
+
+   for key in inbox.iterkeys():
+       try:
+           message = inbox[key]
+       except email.Errors.MessageParseError:
+           continue                # The message is malformed. Just leave it.
+
+       for name in list_names:
+           list_id = message['list-id']
+           if list_id and name in list_id:
+               # Get mailbox to use
+               box = boxes[name]
+
+               # Write copy to disk before removing original.
+               # If there's a crash, you might duplicate a message, but
+               # that's better than losing a message completely.
+               box.lock()
+               box.add(message)
+               box.flush()         
+               box.unlock()
+
+               # Remove original message
+               inbox.lock()
+               inbox.discard(key)
+               inbox.flush()
+               inbox.unlock()
+               break               # Found destination, so stop looking.
+
+   for box in boxes.itervalues():
+       box.close()
+