12.14.1 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.  If no more messages are available,
this method returns 
None.
 
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