PTR and FCrDNS
A PTR record maps an IP address to a hostname; forward-confirmed reverse DNS (FCrDNS) is the check that the hostname resolves back to the same address. Receivers use it to distinguish configured mail infrastructure from arbitrary hosts.
A PTR record answers “what is this IP address called”. It lives in the reverse DNS tree under in-addr.arpa and is controlled by whoever holds the IP block, which is your hosting provider rather than you.
FCrDNS, forward-confirmed reverse DNS, is the round trip: take the IP, find its PTR name, resolve that name, and confirm it comes back to the same IP. Anybody can point a name at an address they do not own, so the forward confirmation is what makes the answer mean something.
Mail receivers treat a sending IP that fails this as unconfigured infrastructure, and it is one of the explicit requirements the large providers publish for bulk senders. RFC 1912 section 2.1 has asked for consistent forward and reverse records since 1996.
If you send through a provider, this is their responsibility and it is usually already correct. If you run your own mail server, it is a support ticket to whoever gave you the IP.
Related
- SPFSender Policy Framework: a DNS TXT record listing the servers authorised to send mail using a domain in the SMTP envelope sender. Defined in RFC 7208.
- DMARCDomain-based Message Authentication, Reporting and Conformance: a DNS record that requires SPF or DKIM to pass for the visible From domain, and instructs receivers what to do when neither does. Defined in RFC 9989.
- Why your mail lands in spam, in the order worth checkingDeliverability problems have a small number of causes and a reliable order to work through them. Authentication first, because it is the only part you can prove.
- Reverse DNSDo your sending IPs resolve back to a name, and does that name resolve forward again?
Every fix on this site is yours to implement. If you would rather someone did it, I take this work directly.