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12.4.1 Mailbox Objects

All implementations of mailbox objects are iterable objects, and have one externally visible method. This method is used by iterators created from mailbox objects and may also be used directly.

`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 `rfc822.Message` object (see the `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`.