Postfix relay problem
 

I can use all the help on this one before I go crazy. I have postifix setup and working. I can send messages to the system from inside our firewall just fine, but when I try sending a message from outside, I receive this:

The original message was received at Fri, 12 Dec 2003 20:07:25 -0500 (EST)

----- The following addresses had permanent fatal errors ----- user@listserv.twu.edu

(reason: 550 5.7.1 Unable to relay for user@listserv.domain.edu)

----- Transcript of session follows -----

... while talking to listserv.domain.edu.:

>>> RCPT To:user@listserv.domain.edu

<<< 550 5.7.1 Unable to relay for user@listserv.domain.edu

Yes, the firewall port is on, and it appears to reach the system, but won't deliver the message. I have been looking online, and through the documentation, but am coming up zeros. Hoping that someone can point me in the right direction. Currently running postfix 2.0 on RH9. Come on gurus.