FCrDNS
Forward-confirmed reverse DNS: the IP address's PTR name must itself resolve, through A or AAAA records, back to the same IP address.
What it means
A PTR record alone proves little, since whoever controls the address can point it at any name. The forward check closes the loop: look up the PTR name's A or AAAA records and confirm the original address is among them. RFC 8601 defines the result as iprev in Authentication-Results.
Google's sender guidelines require it in substance: the sending IP must have a PTR record, and the host name must resolve back to that same IP. After moving a server, check that the PTR record names the new host and that the host's A or AAAA record points to the new address.
Test both directions: IP to name, then name to IP.
Example
192.0.2.25 -> PTR mx1.example.com -> A 192.0.2.25 (confirmed)Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 8601: Message Header Field for Indicating Message Authentication Status, section 3rfc-editor.org
- Google: Email sender guidelinessupport.google.com
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