Adding new domain to MS

[MW Mike Weiner (5028)]

OK, dumb question here but its early and I have had very little coffee this
morning, but I am currently running MS/SA for 4 domains, and wanted to add
as a secondary MX a new domain pointing to this spambox for secondary,
primary for this specific domain is an exchange server. But the domain has
no secondary, so I wanted to point the MX 20 to this MS box. Now, I have
added the "users" to the ham and spam delivery rules, none of which have
valid accounts on this box, as its simply a "pass through" filter before
handing off to its smart host, the exchange server. However, I see
occasionally email coming in for valid users in the domain (i.e. on the
exchange server) but seeing the following in the maillog on the spambox:

3820: Mar 20 10:24:09 spambox sendmail[2854]: i2KFO8Vk002854:
<pcamino at ag.com>... User unknown

Now pcamino at ag.com (or Pcamino at ag.com as exchange knows it) IS a valid user
account on the exchange server, but the spambox, acting as secondary MX for
the exchange server doesn't know pcamino as a valid user even though I have
the following rule in my ham/spam delivery rules:

FromOrTo:   pcamino         store deliver

And maillog shows user as above. Any ideas? I want valid users to be scanned
etc and passed on to the exchange servers, this is only acting as a
secondary MX so that we don't loose any email when exchange exceeds its
concurrent connections and begins to defer mail.

Thanks in advance for any suggestions or assistance
Michael Weiner
Senior Systems Administrator/WebOps
AmericanGreetings.com