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12.14.1 Mailbox Objects

All implementations of Mailbox objects have one externally visible method:

`next`()
  Return the next message in the mailbox, created with the optional factory argument passed into the mailbox object's constructor. By defaul 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`.