Our parent company would like us to start using the same internet domain as they do, even though we have completely separate network infrastructures. At first I thought this was impossible but thinking about how mail/spam filtering works I'm wondering if it could be achieved.

So what I'm thinking is that the required domain - let's call it abcd.com for now - points to their mail server (I believe this is some version of Exchange).

Q1: Can Exchange have a list of specific mailboxes that it re-delivers to another host? A bit like a sendmail alias list?

Assuming Exchange can take our list of users and redirect the mail to us, we then add the abcd.com domain to our accepted domains, make it the default and everything gets should now route in via the abcd.com parent companies Exchange server.
We now set the abcd.com Exchange server as the SMARTHOST/ default route for all mail.

Q2: can we configure GroupWise to send emails to the smarthost for the same domain it is configured as its own if the user does not exist? There will obviously be some xxx@abcd.com users in Exchange and some xxx@abcd.com users in GroupWise, anyone GW doesn't know about should be routed back to the parent Domain/Smarthost.

If anyone has other experiences like this please share, would hate to be forced off GW to an Exchange server