Interesting question. Basically, you have a network with two gateways to the Internet. So all you need to do is tell one set of computers to use one gateway, and tell the other set to use the other gateway.
For example, let's say that the DSL routers internal IP is 192.168.0.1 and you have set the other broadband connections internal IP to 192.168.0.21. In that case the easiest thing to do would be to set the default gateway on all of your machines to 192.168.0.1, and they will use the DSL connection for Internet access. Then you go back to your email server and set it's default gateway to 192.168.0.21, and it will route all of it's email via the other broadband connection.
This is assuming that you're using a very flat network topology with a single subnet. But if you have multiple subnets for servers, desktop PCs, etc, then it gets a little more complicated (though the basic theory still holds).
Now if you're using ISA as a proxy for web browsing, and you have the same server running the proxy and mail server, then it gets more complicated. I'm sure that you could probably do some sort of port mapping on ISA to redirect port 80 and 443 traffic (HTTP and HTTPS) out the DSL connection, but since I'm not very familiar with ISA I'm not sure how. In that case you might have better luck in an ISA forum.
