550 5.7.27 This message was blocked because it didn't pass SPF authentication. Gmail requires bulk email senders to authenticate their email with SPF.

Gmail blocked the message because SPF did not pass. Bulk senders have to authenticate with SPF.

421 4.7.27 is the rate-limited twin: SPF authentication did not pass, and Gmail requires bulk senders to authenticate with SPF.

The title is the 550 5.7.27 row on Google's SMTP error list, last updated 18 September 2026 UTC. This is SPF, not the unauthenticated 550 5.7.26 that accepts SPF or DKIM.