Redirect Exim Mail to Non-Existing Email Addresses to Exchange Server

Dear Colleagues!

I've searched plenty of posts about Exim Smart Routers, but I couldn't find my particular application.

As our Exchange is struggling with it's Pop3 connector, I'd like to relay all email that doesn't address a local user for a specific domain to a remote mailserver (exchange). This is explicitly intended to be an alternative to the "default address"/ catchall function. Rather than collecting that "cought" email in an inbox these mails should be rerouted to another server.

for Examplese
sales@mydomain.com (setup locally) stays on our server

unknown@mydomain.com is routed to mx.myotherdomain.com
Would anybody be able to suggest the configuration for a individual domain?

Thanks so much already!