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.
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.
