External relay not working without recipient domain added as accepted domain

Hi,

I am running Exchange 2013 CU5 on a multi-role server. I have to relay a lot of email from the client's ERP system to external recipients. I set up a receive connector as per the MS documentation, ticked the Anonymous Users security setting and ran the following command:

Get-ReceiveConnector "Connector Name" | Add-ADPermission -User 'NT AUTHORITY\Anonymous Logon' -ExtendedRights "MS-Exch-SMTP-Accept-Any-Recipient"

The problem I am having is that the email does not relay unless I add the recipient email domain as an accepted domain. That will not be a problem if it was only a few recipients, but there are hundreds of different recipients with random email domains, so to add them all as accepted domains is not really viable.

Is this behavior by design, or am I missing a trick? Another client running Exchange 2010 with even more recipients for external relay worked fine after I set up their anonymous relay connector.

Any ideas will be appreciated!

Thanks!

Thomas