550 5.7.25 This message was blocked because the sending IP address doesn't have a PTR record, or the forwarding DNS entry doesn't reference the sending IP address.

Gmail blocked the message because the sending IP has no PTR, or the name in that PTR does not point back at the IP. Gmail requires a PTR on sending IPs.

451 4.7.23 is the rate-limited twin of the same missing or mismatched PTR.

The title is the 550 5.7.25 row on Google's SMTP error list, last updated 18 September 2026 UTC. The page says Gmail requires that sending IP addresses have a PTR record.