Exchange transport rule to silently ignore messages sent to a bogus address

I know you said that you changed the rules to be redirect instead of forward and it didn't resolve the issue; however, redirect is the correct action if you want to preserve the original sender.

What you might want to try is making the redirect change and implementing as transport rules on your hub or edge transports. In fact if you implement on either the hub or edge and it doesn't help you might want to try implementing on the other. Sometimes you will see different behavior depending on whether the transport rule is implemented on the hub or edge transport.

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.