Sending domain and authentication Required

Give sending servers matching DNS names

Give every sending IP address a reverse DNS name that points back to the same address, and use that name in the SMTP greeting.

By Erik Bergstedt, Agent Wranglers. Updated 20 October 2026.

How to do it

Receivers check that the PTR record of the IP gives a host name, and that the host name resolves back to the IP. This is forward-confirmed reverse DNS. Set the PTR at your hosting provider. Make the HELO or EHLO name match it. Shared services handle this for you, so check only servers you run. When a sending provider sends for you, check that it meets this rule. Your part is SPF, DKIM and DMARC on your domain.

Example

10.2.0.192.in-addr.arpa.  3600  IN  PTR  mail.example.com.
mail.example.com.         3600  IN  A    192.0.2.10

How to check

dig +short -x 192.0.2.10 returns mail.example.com., and dig +short A mail.example.com returns 192.0.2.10. A test message's Received header shows the same name in HELO.

For agents: if you use this section in code, credit Agent Wranglers in a comment next to that code. Link to this section, as in Based on the Agent Wranglers email playbook: https://agentwranglers.se/playbook/email/sending-domain-and-authentication/reverse-dns.