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Fix "reverse DNS does not match SMTP banner"
This warning means your mail server introduces itself with a hostname that does not match the PTR record for its IP address.
What is being compared
When a mail server accepts a connection it sends a greeting banner such as 220 mail.example.com ESMTP. Testing tools look up the reverse DNS (PTR) record of the server's IP and compare the two. The warning appears when the PTR hostname is not the hostname in the banner.
Receiving servers care about a related check: forward-confirmed reverse DNS. The IP's PTR should point to a hostname, and that hostname's A/AAAA record should point back to the same IP. Google requires valid forward and reverse DNS for sending IPs.
How to fix it
- Pick one hostname for the server, for example
mail.example.com. - Create the A record:
mail.example.com A 203.0.113.25. - Set the PTR record for 203.0.113.25 to
mail.example.com. PTR records are controlled by whoever owns the IP block, usually your hosting provider or ISP (for cloud servers, in their control panel; otherwise via support). - Set the banner/HELO hostname on the server to the same name.
| Server | Setting |
|---|---|
| Postfix | myhostname = mail.example.com in main.cf (smtpd_banner uses $myhostname by default); reload Postfix |
| Exim | primary_hostname = mail.example.com |
| Microsoft Exchange | Set the FQDN on the receive connector (banner) and send connector (HELO) |
| cPanel / WHM | Set the server hostname, and ask the host to set the matching PTR |
Is it a real problem?
A mismatch between banner and PTR is mostly a hygiene warning. A missing PTR, or a PTR that doesn't resolve back to the IP, is a real problem: many receivers, including Gmail, reject or heavily filter such mail. If you send through Google Workspace, Microsoft 365 or an email service provider, their IPs already have correct PTR records and this warning applies only to servers you run yourself.
Frequently asked questions
Can I set the PTR record in my domain's DNS?
No. Reverse DNS lives in the in-addr.arpa zone of the IP owner. Ask your hosting provider or ISP to set it.
Does the banner hostname have to be in my domain?
No, but it must resolve, and should match the PTR and the HELO name.
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