Enhanced SMTP status code · RFC 3463 · IANA
5.7.10 Encryption needed
This authentication mechanism needs a privacy layer, for example TLS started before AUTH (RFC 5248).
- 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.
How 5.7.10 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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523 5.7.10
SMTP protocol violation, no commands allowed to pipeline after STARTTLS.
The client sent commands right after STARTTLS, without waiting for the TLS negotiation (RFC 3207).
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026