5.1.10: Recipient not found (Microsoft)
Exchange Online returns 5.1.10 when an SMTP address lookup cannot find the recipient. For @hotmail.com and @outlook.com addresses, Microsoft returns the similar 550 5.5.0 "mailbox unavailable". Either way the address does not exist, so treat it as a hard bounce and remove it.
The exact messages
Microsoft 365 / Exchange Online
5.1.10 Recipient not found
Outlook.com / Hotmail
550 5.5.0 550 5.5.0 Requested action not taken: mailbox unavailable
Why it happens
- The address is misspelled or no longer exists.
- An Outlook.com or Hotmail account that was closed or never created.
How to fix it
- Remove the address and verify the rest of the list before resending.
If you send cold email
Old or scraped lists are full of closed Hotmail and Outlook.com addresses. Verify before sending rather than letting bounces do it.
How WarmHawk handles it
A hard bounce marks that lead as bounced and stops its sequence, and every bounce counts toward the mailbox’s bounce-rate breaker: at 5% over at least 20 sends, WarmHawk pauses the mailbox before a stale list can damage the domain. How WarmHawk works →
Check your domain now
These free checkers read your live DNS: no account, up to 15 domains at once.
Related bounce codes
Sources, checked 2026-09-29: Microsoft Learn: NDRs and SMTP errors in Exchange Online. Have a different bounce? Paste it into the decoder →
Questions
5.1.10 questions
What does 5.1.10 mean?+
Exchange Online returns 5.1.10 when an SMTP address lookup cannot find the recipient. For @hotmail.com and @outlook.com addresses, Microsoft returns the similar 550 5.5.0 "mailbox unavailable". Either way the address does not exist, so treat it as a hard bounce and remove it.
Is 5.1.10 a temporary or permanent error?+
Permanent. The message will not be retried, and sending it again unchanged will fail the same way until the cause is fixed.
How do I fix 5.1.10?+
Remove the address and verify the rest of the list before resending.