Exchange 2003 on SBS 2003 - Stopping 3rd Party Spam Filtering - Restoring Inbound SMTP

Hello,

An Exchange server (Exchange 2003 on SBS 2003) that I administer will be discontinuing 3rd Party Spam Filtering (Like GFI, Postini).

In my tests of changing the MX record, I have not been successful in getting inbound email to be restored, to come through. Last year, before using a 3rd party spam filter inbound mail worked swell.

MX was:
10 abccompany.com.pri-mx.smtproutes.com     (1/2hour - for testing)

MX for testing:
10 mail.abccompany.com     (1/2hour - for testing)

So when I remove "abccompany.com.pri-mx.smtproutes.com" no mail flows through Exchange, all users are affected.
When "abccompany.com.pri-mx.smtproutes.com" is in place mail flows fine.

mail.abccompany.com resolves correctly to the static IP for the router and TCP Port 25 forwards from the router to Exchange 2003 on SBS 2003 IP o.k. Port 25 is open on the Windows firewall.

My thought is that there is something in Exchange impeding the inbound mail flow.
The only settings I am aware of affecting the 3rd party spam filtering are:
1. SMTP Connector, Access, Connection and Relay, allowing *.smtproutes.com

Any thoughts?

Sean