Hybrid mail flow with Exchange 2003

I was working with a customer who had Exchange 2010 and Exchange 2003 wanting to move to Exchange Online. We successfully setup the Hybrid Configuration Wizard with Exchange 2010 and were able to move mailboxes to Exchange Online. There was now an issue with mailboxes on Exchange 2003 unable to send messages to mailboxes that had migrated to Exchange Online. They would receive an NDR:

Your message did not reach some or all of the intended recipients. A configuration error in the e-mail system caused the message to bounce between two servers or to be forwarded between two recipients. Contact your administrator. #5.4.6

We found the following error in the Exchange 2003 application logs:

Event Source: MSExchangeTransport

Event Category: NDR

Event ID: 3020


A non-delivery report with a status code of 5.4.6 was generated for recipient {recipient} Message-ID {Message-ID}.

Cause: A forward loop was detected by the categorizer. This is a common hosting configuration problem caused when someone uses the provisioning tool to create a contact in one organization unit and creates a user in a different organization user that share the same e-mail address.

The resolution was to change the Accepted Domain for tenant.mail.onmicrosoft.com from Authoritative to Internal Relay:

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