How can I get Exim to deliver certain domains to a different SMTP port on my local host?

You must set up a special smtp transport, where you can specify the port option, and then set up a router to route the domains to that transport. There are two possibilities for specifying the host:

1. If you use a manualroute router, you can specify the local host in the router options. You must also set

self = send

so that it does not object to sending to the local host.

1. If you use a router that cannot specify hosts (for example, an accept router with appropriate conditions), you have to specify the host using the hosts option of the transport. In this case, you must also set allow_localhost on the transport.