David, i'm going down now.  I'll raise the type to 'crash', because, in fact, EMail 5.1 doesn't really take care of header.Header objects in message.Message headers, which doesn't sound pretty useful to me!  The patch is sufficient for my broken thing (it doesn't produce a traceback at the moment - time to go for a sunday!), but since i don't really have a glue of mailbox.py / email/* it may not cover all places where a header.Header may occur but the code in fact assumes a str (and implicit conversion of Header to str doesn't work).