External E-mail with SBS 2003 Exchange Server

Hi

We have Small Bussiness Server 2003 set up at our office and are trying to get each users google and yahoo email to go through exchange server and into there mailbox on the server.
Now we have got this right using PopCon (Third Party External mail to exchange utility) which is working great but the problem we have is that if I send an email from my internal email address (warren@cssmail.com) to an external email address the person recieving the email will recieve it from my domain email account (ie warren@cssmail.com). Is there a way to change the reply address to my google or yahoo email address

Any Help would be great
Thanks