550 5.7.1 Messages missing a valid address in the From: header, or having no From: header, are not accepted.

Gmail blocked the message because the From header was missing or had no valid address.

A separate 550 5.7.1 row says the message is not RFC 5322 compliant because the From header is missing.
Another 550 5.7.1 row rejects multiple addresses in the From header.

550 5.7.1 is used for many other Gmail blocks. This page is only the missing or invalid From-header rows on the SMTP error list, last updated 18 September 2026 UTC. The IPv6 PTR row and the direct-IP row are different bounces.