SMTP virtual server

Please help me understand the differences between the SMTP Virtual setting on the Exchange server and the setting on a IIS SMTP server.

In my two previuos companies, we don’t have a SMTP relay server, but we have a FE and BE server topology. In this topology, is the FE doing the “relay”.

At my current place, since we are a small start-up compay, we have 1 exchange server and a Windows 2003 IIS smtp relay server behind the Microsoft NAT/VPN/firewall server.
My question is, if I move the SMTP server to the DMZ zone, and turn off basic and windows intergatred authentication, will my pop3 users still able to send mail?

Thanks