What is a reverse DNS (PTR) record?
Reverse DNS resolves an IP address back to a hostname through a PTR record, the opposite of a normal forward lookup. Mail servers check it to gauge sender reputation, so a missing or generic PTR record can hurt email deliverability. PTR records live in the special in-addr.arpa and ip6.arpa zones managed by whoever controls the IP block.
How reverse DNS and PTR records work
A PTR record maps an IP address to a hostname, the reverse of an A record. The IP is queried as a name in a special zone: 93.184.216.34 becomes 34.216.184.93.in-addr.arpa, and IPv6 uses ip6.arpa. Critically, PTR records are controlled by whoever owns the IP block, usually your hosting provider or ISP, not by your domain's DNS. To set or change one, you request it from them rather than editing your own zone file.
Why mail servers check reverse DNS
Receiving mail servers run a reverse DNS lookup on a connecting IP to judge whether it is a legitimate mail source. A clean setup has the PTR resolve to a real hostname (mail.example.com) whose forward A record points back to the same IP, called forward-confirmed reverse DNS (FCrDNS). A missing PTR, or a generic one like a string of digits in a broadband provider's domain, signals a residential or unconfigured host and often triggers spam filtering or outright rejection. Proper PTR is essential for any server sending email.
How to read and fix your PTR results
A PTR result shows the hostname an IP resolves to, for example 34.216.184.93 PTR mail.example.com. Confirm it is FCrDNS-valid by checking that mail.example.com has an A record returning the same IP. If the lookup returns nothing, no PTR is set; if it returns a generic provider hostname, the default is still in place. Fix either by asking your hosting provider or ISP to set the PTR to a hostname you control, then publish a matching forward A record so the two agree.