Hi, there:
Along a similar line, I'm trying to do the same thing.
I have a domain that's full of external users that have an email address in a sub-domain of our authoritative domain. All MX records are working properly, and flow is working just fine.
The problem is that each time we work on a user object with an external address, it gets re-stamped with our default address policy.
I am attempting to create an address policy, but it keeps complaining that we don't have an accepted domain. The only problem is that we DO have an accepted domain for that sub-domain, set to external delivery.
Any thoughts?