PTR doesn't match the mail server hostname (FCrDNS)

What's wrong

For a self-hosted mail server, receivers check FCrDNS: IP → PTR → hostname → A/AAAA → the same IP. If there's no PTR, it's a generic one (static-203-0-113-10.provider.net), or it doesn't match what the server says in HELO/EHLO, mail is rejected or goes to spam.

How to fix

1. PTR is set by the owner of the IP — your hosting/ISP (VPS panel or a ticket), not your DNS host.
2. Set PTR to the server hostname, e.g. mail.example.com.
3. Make sure mail.example.com has an A/AAAA record pointing to the same IP.
4. Use the same hostname in HELO (Postfix: myhostname = mail.example.com).

$ dig +short -x 203.0.113.10
mail.example.com.
$ dig +short mail.example.com A
203.0.113.10

IPv6 needs its own PTR, otherwise Gmail rejects mail over IPv6. If you send through Google/Microsoft/an ESP, PTR is their job — this finding applies to your own SMTP only.

Check it in Email DNS Checker, PTR section.