What is reverse DNS (PTR record)?

Reverse DNS is the lookup that maps an IP address to a hostname, using a PTR record in the in-addr.arpa (IPv4) or ip6.arpa (IPv6) zone. Mail receivers check it at connection time: a sending IP with no PTR record, or whose PTR name does not resolve back to the same IP, is treated as unlikely to be a legitimate mail server. The two-way check is called forward-confirmed reverse DNS (FCrDNS).

The lookup

For 203.0.113.25 the receiver queries:

25.113.0.203.in-addr.arpa.  PTR ?
→ mx1.example.net.

Then it resolves mx1.example.net forward. If the A record includes 203.0.113.25, the reverse DNS is forward-confirmed. If the PTR is missing, or the forward lookup gives a different address, the check fails.

Who controls it

The PTR record lives in the zone for the IP block, which belongs to whoever allocated the address: the hosting provider, ISP or cloud platform. You set it in their control panel, not in your own domain’s DNS. Cloud providers usually require the forward A record to exist first.

Why receivers care

Compromised home computers and cheap virtual machines rarely have a matching PTR record; real mail servers almost always do. Large receivers refuse or heavily penalise mail from IPs without forward-confirmed reverse DNS. Gmail’s sender guidelines state it as a requirement.

A generic PTR such as 203-0-113-25.dynamic.isp.example is also a signal. It says the address is dynamically assigned, which is a strong indicator of a machine that should not be sending mail directly.

Reverse DNS at the handshake

Reverse DNS is known before the sender issues a single SMTP command, which makes it one of the cheapest rejection criteria. Spamjadoo checks it at connection time, compares it with the name given in EHLO, and applies your policy: reject, greylist, or add weight, per recipient class.

Last reviewed 10 September 2026 by Spamjadoo engineering.