Routing mail by Sender domain (not just recipient)

This could potentially be an easy question and i will feel stupid asking it but then again it might just not be possible to do in Exchange.

I am using Exchange 2003 and have a bunch of different email addresses (from user's ISP accounts) forward to my mailserver. This works fine and the users get their home email. Then, if they want to send email out from their home email account they "Send As" their home email address that i have entered into the Active Directory as a contact with them given Send As Permisions. In essence i am allowing my exchange server to spoof mail coming from their email address. This works great but i am concerned that as new SPF anti-spam policies become prevalent i need to really get mail sent from their ISP's.

Therefore, i want to be able to route all mail coming from a sender with lets say "@mindspring.com" to go to Mindspring's Mail server (and then i would use the AUTH feature to allow me to relay mail through mindspring). The only thing i cannot do right now is get Exchange to route based on the sender of the mail, not the recipient. I have never thought this was possible but i heard someone once say it was, to my disbelief. Anyone have any answers (and i challenge you all to make me feel stupid)?

Thanks a bunch,

Chris

Also, didn't really know how many point to award for this so if it isnt enough i will boost it later.