Apps :: Sending with Sylpheed

Can anyone give me a clue about a "501 unacceptable mail address" error when sending mail in sylpheed? I have checked, double checked and triple checked my MS outlook settings and compared them to sylpheed and they seem to match exactly. I even brought up mozilla mail in Knoppix 3.4 and setup the account without issue. I can't seem to find mention of this error anywhere.

Any help is greatly appreciated! Thanks

Chris

original here.