How to use new email package to read a mailbox?

[Andrew Koenig]

The documentation for the mailbox package says that the default behavior
of class UnixMailbox is to use rfc822.Message to create messages.
However, rfc822 is noted as deprecated.

Suppose I have a file that represents a Unix mailbox.  What's the
right idiom to use to process each message in that file using the
new email package?

-- 
Andrew Koenig, ark at research.att.com, http://www.research.att.com/info/ark