550 5.1.10 Recipient not found
Meaning
The recipient's email address could not be found by the receiving server's directory lookup.
Common causes
- The email address does not exist in the recipient's organization.
- The recipient's mailbox was recently deleted or migrated incorrectly.
- You are sending to a legacy address that is no longer mapped to a valid user.
How to fix
- Treat this as a hard bounce and remove the recipient from your active sending list.
- If you know the user, contact them via another method to get their updated email address.
- Do not retry sending, as it will negatively impact your sender reputation.
Provider notes
Microsoft 365. Common in Exchange Online when the recipient's SMTP address wasn't found by directory lookup.
Example bounce
550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup.
FAQ
- Is this different from 550 5.1.1?
- Functionally, no. It is Microsoft Exchange's specific enhanced code for an invalid user. Handle it exactly like a 5.1.1 hard bounce.