Glossary · DNS
A PTR record maps an IP address back to a hostname. Receivers expect every mail-sending IP to have one that matches its forward DNS.
Forward DNS turns a name into an IP; reverse DNS, stored under in-addr.arpa for IPv4, turns the IP back into a name. Forward-confirmed reverse DNS means the name resolves back to the same IP.
Gmail and other providers reject or junk mail from IPs without valid reverse DNS. The record is set by whoever owns the IP (your hosting or mail provider), not in your domain's DNS. If you use a hosted mail service, they handle it.
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