Create Exchange 2007 Transport Agent

I've been tasked with creating an Exchange 2007 Transport agent to route emails from given domains through different firewalls.  We currently have two domains that send mails to our exchange server, (my-domain.com and mydomain.com).  If an email is sent from the first, it needs to go out from firewall X but if sent from the second it needs to go out from firewall Y.

I found this resource: http://msexchangeteam.com/archive/2006/12/04/431755.aspx but unfortunately it doesn't give me enough information to really even begin this development process.  Specifically, I'm not sure whether I should be using the routing agent or the smtp agent (or potentially both).  After reading that article, it sounds like I need the SMTP agent to determine the origin domain of the email but the routing agent to 'route' my mail to the appropriate firewall.

I'm really at a loss here and would sincerely appreciate any guidance that you can offer.  At this point I'd really just like to get my feet off the ground but as stated above, I'm not even sure where to begin to do that....

If this was posted in the wrong forum, I apologize.  This is my first visit to this community and I only saw one forum in which to post.

Burr