Scenario: After standing up a new Exchange On-Premises Server, users are receiving the bounce back message with wording similar to the following:

For Email Admins: The message couldn’t be sent because it’s an attempt to relay a message to a recipient in a non-accepted domain (open relay) which isn’t allowed.

-or-

‘550 5.7.54 SMTP; Unable to relay recipient in non-accepted domain’

Solution: Make sure the Default Frontend Receive Connector is set to accept AnonymousUsers when connecting AND the ADPermission for AnonymousLogon is applied to the Receive Connector on the new server:

Set-ReceiveConnector “ExSrv1\Default Frontend ExSrv1” -PermissionGroups AnonymousUsers

Get-ReceiveConnector “ExSrv1\Default Frontend ExSrv1” | Add-ADPermission -User ‘NT Authority\Anonymous Logon’ -ExtendedRights MS-Exch-SMTP-Accept-Any-Recipient