Enhanced SMTP status code · RFC 3463 · IANA
5.7.11 Encryption required for the mechanism
The chosen authentication mechanism may only be used over an encrypted connection (RFC 4954).
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1).
- Subject: X.7 · Security and policy
Policies (per-host or per-recipient filters) and cryptographic operations; up to the sender, the recipient or both.
- Associated basic codes (IANA registry)
How 5.7.11 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Gmail
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501 5.7.11
Syntax error (no parameters allowed).
STARTTLS was sent with parameters, and RFC 3207 allows none.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026