anyone having problems with SMTP auth?

Hi all, My SMTP auth works fine with Mozilla thunderbird (using system account), but fails with a:

*SMTP Error in /opt/web/roundcube/program/steps/mail/sendmail.inc (179):* Connection failed: 554 : Relay access denied

No valid recipients

in roundcube. Anyone else running into this problem? Have workarounds?

I have my SMTP domain, username and password set in the config file - seems like it could be nice to have this keyed off of the end-users username:password also, is there any way to store these in the session and pull them out for this purpose, or is that generally frowned upon?
(I always want to do this for "single sign on" across my various PHP apps anyway)

thanks for any info, j.