Subject does most of the work here, but I'll expand. [edit] Maybe if I had just RTFM'd harder I would've found my answers... I think this will help, but leaving the post up in case anybody else has ...

I'm attempting to set up Postfix to act as a client to Exchange 2003. I have the relaying set up, I have SASL set up, I have TLS set up, etc. If I do the same account setup in a mail client, it works ...