Microsoft 365 450 4.4.317
Cannot connect to remote server [Message=UntrustedRoot]
Microsoft 365 450 4.4.317 means: cannot connect to remote server [Message=UntrustedRoot]. 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
Cannot connect to remote server [Message=UntrustedRoot]
Possible cause
During the Transport Layer Security (TLS) handshake, Exchange Online can't verify the authenticity of a leaf certificate sent without the full certificate chain by a remote email server.
| Who sends it | Microsoft 365, Exchange Online and Outlook.com |
|---|---|
| Basic code | 450: Requested mail action not taken: mailbox unavailable (e.g., mailbox busy or temporarily blocked for policy reasons) (RFC) |
| Enhanced code | 4.4.317: Microsoft 365-specific, not in the IANA registry. Its subject digit puts it under 5.4.0 other or undefined network or routing 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 365 450 4.4.317?
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 365 450 4.4.317 mean?
- Microsoft 365 450 4.4.317 means: cannot connect to remote server [Message=UntrustedRoot]. 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 365 450 4.4.317?
- 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 365 450 4.4.317?
- 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)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (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