Exchange Online Email delivery

550 5.1.1: the recipient's address doesn't exist (bad destination mailbox address)

Likely causes

• The address is misspelled.
• The recipient's address was changed or removed.
• A stale entry in the Outlook Auto-Complete list points to a mailbox that moved.
• A forwarding rule on the recipient's side sends a copy to an address that no longer exists.
• The recipient domain's MX record points to the wrong or an old mail server.

How to fix it

1. Check the address in the bounce against the address you meant to use. Fix any typo and send again.
2. Remove the recipient from the Auto-Complete list: start typing the name in the To box, highlight the suggestion, press Delete, then type the full address by hand. Resending the original message gives the same failure.
3. If the address in the bounce doesn’t match the one in your Sent Items, ask the recipient to check for a forwarding rule pointing at a bad address.
4. Check your Sent Items for messages you didn’t send. If you find some, reset your password and tell your admin: the account may be compromised.
5. Admins: check Health > Service health in the Microsoft 365 admin center, confirm the recipient exists, and confirm the domain has one MX record that points to Exchange Online (<domain>.mail.protection.outlook.com, or <domain>.<subdomain>.mx.microsoft on newer setups).

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