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Error codes

550 5.7.29 This message was blocked because it wasn't sent over a TLS connection.

Gmail blocked the message because the SMTP session was not TLS. Bulk senders have to use TLS/SSL for SMTP.

  • 421 4.7.29 is the rate-limited twin: the message was not sent over TLS, and Gmail requires bulk senders to use TLS/SSL for SMTP connections.

The title is the 550 5.7.29 row on Google's SMTP error list, last updated 18 September 2026 UTC. The rest of that row names TLS/SSL for SMTP connections.