When sending a message from the internet to a user that has been migrated to Exchange you receive an NDR stating "Maximum hop count exceeded. Message probably in a routing loop."
When sending an email to the same user from a Notes Client the message is delivered as expected.
In this environment this was caused by the setting in Domino that controls the "Maximum hop count".
Once this was increased and the Domino Router was restarted messages from the internet were able to be delivered correctly
For more information on this setting please refer to Setting transfer limits from IBM.