550 5.1.10: recipient not found by SMTP address lookup

What it means

Exchange Online returns 5.1.10 when it takes mail for a domain and then finds no recipient with that SMTP address. The address may be wrong or deleted, or the recipient may lack a license or a synchronized directory object. In a hybrid setup, the accepted domain type matters: an authoritative domain makes Exchange Online stop at its own directory, while internal relay passes unknown addresses on to the on-premises organization. A mailbox created in the cloud with no matching on-premises object produces the same reply for on-premises senders. Spammers forging your address can also trigger it as backscatter.

Common causes

• The address is wrong or the recipient was deleted.
• The recipient exists but has no license that includes email.
• A forwarding rule on the recipient side sends to an address that no longer exists.
• In a hybrid deployment, the recipient is not synchronized, or the accepted domain is authoritative where internal relay is needed.

How to fix it

1. 01Check the address, remove it from your Auto-Complete list, type it in full and send a new message.
2. 02If you did not send the original message, treat the report as backscatter, and run a malware scan if you receive many.
3. 03Recipient admins: confirm in the Microsoft 365 admin center that the user exists and has a license that includes email.
4. 04Remove or correct inbox rules, account forwarding and mail flow rules that redirect to bad addresses.
5. 05In hybrid setups, run a delta sync (Start-ADSyncSyncCycle -PolicyType Delta) and check the accepted domain type; for a cloud-created mailbox, create its remote mailbox on premises.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

• Fix NDR error "550 5.1.10" in Exchange Onlinelearn.microsoft.com
• "550 5.1.10 RESOLVER.ADR.RecipientNotFound" when Microsoft 365 users send emails to on-premises users in a hybrid deploymentlearn.microsoft.com
• Email nondelivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com

Further reading

• The standard meaning of 556 5.1.10: recipient domain has a null MX