I've got a mail server configured to send mail directly (not via ISP mail server).
All is OK and I have set reverse DNS with ISP which works. So:
Mail server: mail-server.mydomain.com
Sends via public IP: 220.127.116.11
Which when checked for reverse DNS returns mail-server.mydomain.com
which is configured via DNS to IP address 18.104.22.168
All is fine, but we have recently added a failover router that allows us to use an alternative broadband connection (different ISP) in case of problems.
In that case, the mail server will send via Public IP: 22.214.171.124. I have created a new DNS entry for this IP in our DNS as mail2.mydomain.com and configured Reverse DNS with this ISP so Public IP: 126.96.36.199 reverts to mail2.mydomain.com. However, the mail server is still called mail-server.mydomain.com.
So my question (finally!) - does the reverse DNS test used as a Spam filter by some ISPs & Mail Servers use the Mail Server name in the test at all - or does is simply use the sending IP.