Hm, it seems right now that the only time that happens is when creating a message based on an mbox message.  The 'status' and 'x-status' attributes are deleted.

Any hints on what I could to do figure out what special attributes should be deleted?  I was thinking I might try to find and go through the mailbox specs, and find out what headers are mandatory for each of them.

I think I see some hints on... http://docs.python.org/library/mailbox.html#mailbox.mboxMessage.