Resolve an IP address back to its PTR / reverse-DNS hostname and forward-confirm the match — the FCrDNS check that mail servers and allowlists rely on.
Try
🎓Learn: reverse DNS
What is a PTR record?▾
The opposite of an A/AAAA record: it maps an IP address back to a hostname. It lives in the special in-addr.arpa (IPv4) or ip6.arpa (IPv6) zone, controlled by whoever owns the IP block.
What is FCrDNS (forward-confirmed reverse DNS)?▾
A trust check: the PTR hostname must also resolve forward back to the original IP. Mail servers and allow-lists treat unconfirmed reverse DNS as suspicious, so a mismatched or missing PTR often means your mail lands in spam.