Subdomains directing to different web servers behind single public IP address

I have two separate web servers with two different internal IP addresses on a network with only one public IP address. One is Windows Small Business Server 2011 and the other is a new e-mail encryption gateway appliance.

I have subdomains configured for remote.domain.com and securemail.domain.com both pointing to the same public IP. I need both of these subdomains to point to the correct internal server both on port 80.

I've configured forward lookup zones on the SBS server pointing to the appropriate local IP addresses. I have a SonicWall NSA 220 firewall. I have firewall rules to redirect 80 (and other services) to the SBS server. I've set up an overriding rule to redirect 80 to the gateway appliance, but this adversely affects Exchange and other SBS services.

Thanks in advance!