Exchange Online Email delivery

550 5.1.10: recipient not found by SMTP address lookup

Likely causes

• The address is wrong or the mailbox was deleted.
• A bad Auto-Complete entry is being reused.
• A forwarding rule on the recipient's mailbox points to an address that doesn't exist.
• Hybrid setups: the on-premises recipient isn't synced to Exchange Online, or the domain is set to Authoritative when it should be Internal Relay.
• Backscatter: a spammer forged your address, so you got a bounce for a message you never sent.

How to fix it

1. Check your Sent Items. If you never sent the original message, the bounce is backscatter and you can delete it.
2. Otherwise, delete the recipient from the Auto-Complete list, type the full address again carefully, and resend.
3. Ask the recipient whether an Inbox rule or mailbox forwarding sends your mail somewhere else.
4. Admins: in the Microsoft 365 admin center, confirm the recipient exists under Users > Active users and has a license that includes Exchange Online.
5. Admins: check Mail flow > Rules in the Exchange admin center for redirect rules pointing at bad addresses.
6. Hybrid admins: run Start-ADSyncSyncCycle -PolicyType Delta on the Entra Connect server, then check the recipient again. Only change the accepted domain to Internal Relay if you have a connector that routes mail to on-premises.

Checked against: learn.microsoft.com, learn.microsoft.com