Routing Services Between LANs

Let’s say I have two LANs. One LAN accesses the Internet via a DSL line hooked up to a router. The other LAN accesses the Internet using an offsite proxy server. The offsite proxy server doesn’t allow SMTP, POP3 or IMAP, but the DSL LAN can use these services fine. Now let’s say I’ve configured Win2K3 Server with one NIC on the DSL LAN and the other on the Proxy LAN. Is there any way to route SMTP, POP3 and IMAP traffic directly through the DSL LAN?