A hectic day and now we use GMime for all MIME-fu.
This has allowed me to decode headers correctly, setup MIME parts properly in outgoing mails with attachments, and cleanup the code-base.
The next release of Lumail will contain basically just this change, as it is pretty drastic. But first I need to work out how to make binaries for Squeeze compiled against the back-ported version of gmime-2.6.x.
(Previously we used libmimetic. Which was awesome in its way, but caused me some pain with RFC 2047 header-decoding.)