If you have to get one DNS record right, the reverse DNS record, also know as a PTR record, is the one. As the name states, these records do the reverse of what a normal DNS A record does — they map IP addresses back into host names.
Without a valid reverse DNS record, many ISPs will block your email .
Here’s a simple example showing a forward, conventional DNS lookup using an A record and a reverse DNS record.
Here’s another example from Microsoft:
Forward A record:
dig mail.microsoft.com
;; ANSWER SECTION: mail.microsoft.com. 681 IN A 131.107.125.5
Reverse PTR record:
dig -x 131.107.125.5
;; ANSWER SECTION: 5.125.107.131.in-addr.arpa. 3600 IN PTR mail.microsoft.com.
In the DNS lookups, you can see that the A record for mail.microsoft.com resolves to 131.107.125.5 and that if we lookup up 131.107.125.5, we get back mail.microsoft.com. There is no requirement that the forward and reverse lookups must match but it is helpful if they do.
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