Can bordermanager do this?

Situation is this.. I work for a medium sized business with a larger parent business.

Larger business has cisco firewall IOS router in front of bordermanager proxy. We have registered a domain name and would now like to configure mail such that we have users@ourdomain.com however bordermanager is already acting as a e-mail proxy for theirdomain.com mail addresses. The larger company IT staff tell me that Bordermanager can only act as a mail proxy for 1 domain – Is this true – if not how do you configure Bordermanager for multiple e-mail domain proxy???