Error codes

550 5.7.1 This message does not meet IPv6 sending guidelines regarding PTR records and authentication.

Gmail blocked the message because the IPv6 sending path did not meet its PTR and authentication rules.

550 5.7.1 is used for many other Gmail blocks. This page is only the IPv6 PTR-and-authentication row on the SMTP error list, last updated 18 September 2026 UTC. The other 5.7.1 texts are not this bounce.