Route some email addresses to remote email server using Exim routes

We would like to use Proton Mail for some personal email boxes, but keep using ScalaHosting for other email boxes. Since MX records can only specify one receiving email server, this requires routing messages to some email addresses to the Proton Mail email server, while processing/storing the other messages locally. We believe Exim, which is currently used for email processing, already offers this functionality. We think this is a reasonable scenario that might also benefit other users, and that promotes email modularity and independence.