Now, some mail servers are more forgiving than others and look only to see if the IP matches, but I have had mail servers be so picky as to make sure not only the IP matches, but also the host name. And, I've found it happens more often than not lately.
An IP having a PTR record is insufficient, because, like any DNS record, the RDATA field can be anything, forged.
Therefore, any serious reject_unknown_client test will query for the PTR domain name, and then query for that domain name's A record: PTR + A must match.
Len
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