Microsoft 365 4.7.321
starttls-not-supported: Destination mail server must support TLS to receive mail
Microsoft 365 4.7.321 means: starttls-not-supported: Destination mail server must support TLS to receive mail. It is a temporary failure, so your server retries it. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
Microsoft 365’s exact wording
Quoted from Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online, checked 2026-11-05. Placeholders are Microsoft 365’s own.
Provider's description
starttls-not-supported: Destination mail server must support TLS to receive mail.
Possible cause
DNSSEC checks passed. However, upon establishing the connection, the destination mail server doesn't respond to the STARTTLS command. The destination server responds to the STARTTLS command, but the TLS handshake fails.
Provider's advice
This message usually indicates an issue on the destination email server. Check the validity of the recipient address. Determine whether the destination server is configured correctly to receive the messages.
| Who sends it | Microsoft 365, Exchange Online and Outlook.com |
|---|---|
| Basic code | Microsoft 365 lists the enhanced code only |
| Enhanced code | 4.7.321: Microsoft 365-specific, not in the IANA registry. Its subject digit puts it under 5.7.0 other or undefined security status. |
| Permanent or temporary | Temporary (4xx) |
| Verifying the list prevents it | No |
What the sender should do
The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
Would verifying the list have prevented Microsoft 365 4.7.321?
No
This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Common questions
- What does Microsoft 365 4.7.321 mean?
- Microsoft 365 4.7.321 means: starttls-not-supported: Destination mail server must support TLS to receive mail. It is a temporary failure, so your server retries it. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
- Should I retry after Microsoft 365 4.7.321?
- Your sending server retries it automatically, because it is a temporary (4xx) failure. Investigate only if it keeps coming back over several hours.
- Would verifying my list have prevented Microsoft 365 4.7.321?
- No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Sources
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05