##
12.17.1 Mailbox Objects

All implementations of Mailbox objects have one externally visible method:

`next` ()
  Return the next message in the mailbox, as a `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.