A common practice on the net is for mail servers, before accepting email from other email servers, to run an RDNS check on the sending server's IP address. This is one way to control SPAM because it at least ties a domain name to an IP.
The problem is, although we run pretty good Anti-SPAM, we still have some really annoying SPAM coming through the system. In part because senders are spoofing our internal email addresses so that the SPAM email looks like its coming from someone on the inside. Our Anti-SPAM filters are configured to ignore internal mail...
Also the outgoing queue is plugged up sending NDR replies from email@example.com to email addresses that do not exist because the SPAMMER spoofed them.
Bottom line, I think that doing an RDNS check on sending servers has become a standard enough practice that we should implement it. We have a Windows 2003 SBS Std server...
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