PTR record
The DNS record that maps an IP address back to a host name, published in the in-addr.arpa or ip6.arpa reverse zone by whoever controls the address.
What it means
A PTR record for 192.0.2.25 lives at 25.2.0.192.in-addr.arpa and names a host, such as mx1.example.com. IPv6 addresses use ip6.arpa with each hex digit reversed. Because reverse zones belong to whoever holds the address block, PTR records are set through your hosting or IP provider, not at your domain's DNS host.
Mail receivers check them. Google and Yahoo require sending IP addresses to have valid PTR records whose names resolve back to the same address; Yahoo adds that names should be meaningful rather than generic dynamic-looking ones. Missing PTR records are a standard reason for 4.7.0 deferrals at Gmail and rejections elsewhere.
Giving each sending IP one PTR record whose name matches the server's HELO name keeps the two checks consistent.
Example
25.2.0.192.in-addr.arpa. 3600 IN PTR mx1.example.com.Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 1035: Domain Names: Implementation and Specification, section 3.5rfc-editor.org
- RFC 1035: Domain Names: Implementation and Specification, section 3.3.12rfc-editor.org
- Google: Email sender guidelinessupport.google.com
- Yahoo Sender Hub: Sender best practicessenders.yahooinc.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes