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user_id=12800

I don't have time to comment except to say that
email.message_from_file() definitely works as the factory
function for mailbox.UnixMailbox.  As proof, I have the
following code in Mailman:

class Mailbox(mailbox.PortableUnixMailbox):
    def __init__(self, fp):
        mailbox.PortableUnixMailbox.__init__(self, fp,
email.message_from_file)

so I know it works. :)

I'll look at the comments Paul's made when I get a chance. 
I'm assigning this bug report to me.