We currently have 1500 users on two Exchange 2003 servers (two sites) with one Front-end server. We out-source all of our anti-virus and anti-spam to surfcontrol. So, we have no MX records pointing to our Front-End server, only our surfcontrol service. We are going to deploy Exchange 2007, and we have decided that we need to have a pair of CAS role servers in our two sites, and we don't know if we also need a pair of Edge Transport servers too. Is it absolutely vital to have ET servers? Can we just use IIS SMTP relay on the CAS servers? We have absolutely no need for any of the features that ET provides.

Yes you can route all your incoming/outgoing traffic through external SMTP Gateway from Hub Transport Srver. How to Configure Internet Mail Flow Through Exchange Hosted Services or an External SMTP Gateway You must have a Hub Transport Server anda CAS Server in site where Mailbox Server is installed. Here are some of the references for your deployment. Understanding Exchange Server 2007 server roles Planning for a Standard Exchange Organization Best Practices for Transitioning an Exchange Organization