Mail server rejecting some mail - Desperate!

[Bryan O'Neal]

Setup

I have a mail server that I would like to handle several domains.  The
primary domain is working but the secondary domain is not.  The primary
domain has it's MX records pointed to my spam filter when then points to
my mail server (66.193.205.166)  I have the MX records for the other
domain pointing directly to my mail server at mail.cornerstonehome.com
(66.193.205.166)  I pass port 25 from my fire wall to my server.  

Symptoms

Mail sent to the primary domain gets delivered.  Mail sent to the
secondary domain gets rejected (...smtp;550 5.7.1 Unable to relay...)