PMDF System Manager's Guide

Setting mailfromdnsverify on an incoming TCP/IP channel causes PMDF to verify that an entry in the DNS exists for the domain used on the SMTP MAIL FROM: command, and to reject the message if no such entry exists. nomailfromdnsverify is the default, and means that no such check is performed.

Note that performing DNS checks on the return address domain may result in rejecting some desired valid messages (for instance, from legitimate sites that simply have not yet registered their domain name, or at times of bad information in the DNS); it is contrary to the spirit of being generous in what you accept and getting the e-mail through, expressed in RFC 1123, Requirements for Internet Hosts. However, some sites may desire to perform such checks in cases where junk e-mail (SPAM) is being sent with forged e-mail addresses from non-existant domains.