Enhanced SMTP status code · RFC 3463 · IANA
5.7.321 starttls-not-supported: Destination mail server must support TLS to receive mail
The recipient's domain uses DANE, but its server does not answer STARTTLS or the TLS handshake fails.
- 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).
Also seen as:
4.7.321 - 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 5.7.321 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Microsoft 365 / Exchange Online
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5.7.321
starttls-not-supported: Destination mail server must support TLS to receive mail.
The recipient's domain uses DANE, but its server does not answer STARTTLS or the TLS handshake fails. Check with SMTP server check.
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4.7.321
starttls-not-supported: Destination mail server must support TLS to receive mail.
The recipient's domain uses DANE, but its server does not answer STARTTLS or the TLS handshake fails. Check with SMTP server check.
What you can check here
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026