There is also a semi-obscure reference to this situation within the man pages for qpopper , but no direct reference to this message. Basically, qpopper does a reverse DNS lookup on the IP address it it given when you try to read your mail. It then does a DNS lookup on that name to verify that the IP address matches. If it does not, it puts out the above message but continues to process the request. In that respect, the message is only a warning.
Services can continue otherwise unhindered.
July 1998 I've received lots of help from a FreeBSD Questions mailing list person, who shall remain nameless until future notice. He suggested I send him my named files as a starting point. There were many glaring errors, which, when pointed out, were obvious even to me. Some of my reverse IP entries where totally incorrect. I implemented the changes as suggested by my helper.