Error codes

530 5.7.0 Must issue a STARTTLS command first.

Gmail closed the SMTP session because the client did not issue STARTTLS first. The error list points at RFC 3207.

The title is the 530 5.7.0 STARTTLS row on Google's SMTP error list, last updated 18 September 2026 UTC. 530 5.7.0 is also used for Authentication required. That other row is not this bounce. Bulk senders that skip TLS altogether get 550 5.7.29.