So You see this error in NDR when the message bounces back.

In my case I had a normal AD User with mail-attribute populated. Exchange figured out that this user has a mailbox inside the organization.

It wasn’t. The user is located in local AD, but is only synced to Office 365, no local mailbox provisioned.

So the fix is, add TargetAddress -attribute to the users with the same mail-address that they have.

And below is the script for it.

You can also use different filters, but I used UserPrincipalName.

Happy Powershelling,