550 5.1.10: Recipient not found (Microsoft 365 and Outlook.com)
Microsoft's mail system could not find the recipient. It's Microsoft's version of "this mailbox doesn't exist".
Hard bounce The address is bad. Don't keep sending to it.
What the bounce message looks like
The wording changes from one provider to another. These are typical examples:
550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient [email protected] not found by SMTP address lookupCommon causes
- A typo in the address
- The mailbox was deleted or renamed in Microsoft 365
- The domain's MX records point to Microsoft, but the mailbox was never set up there
How to fix it if you sent the email
- Check the address for typos.
- If it's correct, ask the person whether their address has changed.
- Remove it from your mailing list if it keeps bouncing.
If you manage the receiving or sending mail system
- In the Microsoft 365 admin center, confirm the user and the address (including aliases) exist.
- If you recently moved email to Microsoft 365, check the mailbox was created and the domain is verified.
Free tool: paste the full bounce into the bounce message decoder to get a plain-English reading of every code in it.
Questions people ask
- What does 550 5.1.10 mean?
- Microsoft's mail system could not find the recipient. It's Microsoft's version of "this mailbox doesn't exist".
- Is 550 5.1.10 a hard or soft bounce?
- Hard bounce. The address is bad. Don't keep sending to it.
- Should I remove the address after a 5.1.10 bounce?
- Yes. Remove it from your list, or confirm a new address with the person first.