I dunno if this is helpful, but in the 2.5 module, it parses mailboxes
into rfc822 messages, but then expects them to be email.Message messages
when unparsing them back to a mailbox. mbox2.add(mbox1.popitem()[1])
fails with rfc822 as the default factory. Since the "factory" is the
only thing still using rfc822, it's easy to remove the use of rfc822
from this module entirely, which also eliminates the parsing/unparsing
disconnect.