Setting up mail exception filters
Novell Cool Solutions: Question & Answer
Reader Rating 
Q:
We are having a lot of problems with spam. We are trying to create packet filter exceptions that would only allow incoming mail from the ISP's mail server. Could someone tell me how to set up this filter?
A:
You should already have SMTP exceptions that allow e-mail in, probably from all addresses. Simply add the ISP's mail server address as the source IP address for inbound SMTP. Add additional filter exceptions as needed for multiple ISP mail servers. (Or instead of calling out the mail server individual addresses, call out the ISP's mail server subnet instead, if all the servers are on the same subnet).
There are ramifications for MX records here (you probably want the
primary to point to the ISP, and you need to talk with the ISP about
the whole situation).
The Postini (http://www.postini.com) mail filtering service, for example, requires this sort of filtering. Caledonia.net ( (http://www.caledonia.net/iris.html) is another mail filtering service.
Novell Cool Solutions (corporate web communities) are produced by WebWise Solutions. www.webwiseone.com