How you can fix the issue Easiest fix: double-check that the sender typed the correct address and that the recipient still ...

Office 365


How you can fix the issue
Easiest fix: double-check that the sender typed the correct address and that the recipient still exists.
Clear the sender's email AutoComplete cache and retype the correct address. (Read this article for details on how to do this in Outlook.)

If those steps don't work, you need to contact the recipient's email admin to fix the problem. This isn't an issue with your Office 365 organization.

Help the recipient admin fix the issue
If the steps in the NDR didn't work, the recipient's email system is probably misconsigured. Here are some things the recipient's admin can check.

Fix delivery problemsMX record or email address directory issues. For tips on fixing this problem, see Status code 5.1.10 in Exchange Online and Office 365.
Fix mail loop issuesIf the recipient domain is for mailboxes only hosted in Office 365, make sure the accepted domain is set to Authoritative and not Internal Relay.
If the recipient organization is in a hybrid cloud/on-premises configuration, make sure the accepted domain is set to Internal Relay, and an outbound connector for routing messages to the on-premises mail system is correctly set up.

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