Benni,
can you provide the message you are sending to roundup -- if possible
with all headers and mime formatting intact. That way it's easier for me
to test this. You may want to strip the message from any confidential
information it may have (but please verify that you're still seeing the
problem).
You should restore the mailgw.py to its original state before trying my
modification. I'll test my own modification now -- preferrably with your
message.

Thanks, Ralf