Users cant choose a router, they can only arp for things in their network or point to a gateway router to get out of their network. Same thing on the email server's side. You didnt describe the topology so I'm guessing these are the only two routers. it sounds like the 1700 should be the gateway. If the 837 and 1700 are running a vlsm routing protocol have the 837 advertise a host specific route for the email host that is better than any route the 1700 has. That will make the 1700 forward email traffic from the users to the 837.
On the other side the email host should use the 837 as its gateway. If there are more routers involved you need to explain the whole topology.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...