550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup
Sample bounce line
550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup
What it means
Exchange Online's SMTP address lookup did not find the recipient's address, so the message could not be delivered. Microsoft's general page says the recipient's email address doesn't exist or couldn't be found.
Why it happens
- The recipient's email address doesn't exist or couldn't be found (mistyped address, stale Outlook Auto-Complete entry, or a recipient forwarding rule pointing to a bad address).
- Backscatter: a spammer sent to a non-existent recipient and forged your address in the From field, so you receive a useless bounce. Microsoft says you can safely ignore or delete it.
- Hybrid deployment: a user created in Microsoft 365 has no on-premises object, so on-premises lookups for that SMTP address fail (the recipientnotfound-ndr page's scenario).
- Accepted domain is Authoritative and the recipient isn't in the Exchange Online organization (on-premises users must be represented by mail contacts or mail users).
How to fix it
- Sender: check the spelling of the recipient address, remove the stale entry from the Auto-Complete list, and resend.
- Ask the recipient to check for Inbox rules or account forwarding that send to a bad address, and to update or disable them.
- Admin: in the Microsoft 365 admin center (Users > Active users), confirm the recipient exists and has a valid email license.
- Admin: fix or remove broken forwarding (Inbox rules, account forwarding, mail flow rules).
- Admin: review the accepted domain type (Authoritative vs Internal Relay), and for hybrid, make sure a matching connector to on-premises exists.
- Admin: verify the domain's MX record.
- Hybrid, user created in Microsoft 365: create a remote mailbox on-premises with New-RemoteMailbox, set its ExchangeGuid to match the cloud mailbox, then run directory synchronization.
Check your domain now
Many of these errors come from missing or broken SPF, DKIM, DMARC or reverse DNS records. Enter your domain to run the free check.
Related errors
Official documentation
- https://learn.microsoft.com/en-us/troubleshoot/exchange/email-delivery/ndr/recipientnotfound-ndr
- https://learn.microsoft.com/en-us/troubleshoot/exchange/email-delivery/ndr/fix-error-code-550-5-1-10-in-exchange-online
- https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/non-delivery-reports-in-exchange-online/non-delivery-reports-in-exchange-online