Hmm, I think you should consider using relay-ctrl (
http://untroubled.org/relay-ctrl/ ). I'm certaily no mail expert but I think this app would help.
It's been ages since I configured this on my server but I dont't recall it being very hard to deal with (in great part thanks to the invaluable help from Dave Bauer).
FWIW, I made some notes from that session, but they may be obsolete by now: http://infogettable.net/linux/mail-system/
Good luck!
