New-InboxRule to autoreply, or always autoreply

I work in a ~300 person subsidiary of a 20x larger parent company, and we have mail contacts in the parent exchange org configured with a forwarding rule. Since your divestiture is a one-time occurrence, there would not need to be further upkeep with regard to mail contact maintenance if you did something similar.

OOO handling is a single reply for a number of reasons, mostly how easy it would be to cause a forwarding loop if it wasn't a one-time rule.

Given that you're setting up a new Exchange domain (presumably with a new .com domain for email), perhaps you could make a transport rule that returns a message to sender in addition to forwarding in the scenario I described above. Mail Delivery Options work a little differently from a transport rule, so I'm not sure this approach is feasible.

RFC 3463 seems to be designed to be helpful in your case, a status code of 5.1.2 would apply directly (Permanent Failure: Mailbox has moved). Unfortunately you will probably never be able to rely on outside partners parsing that correctly.