if exim is showing up errors like “unrouteable address”, it means that exim is trying to send mail locally. If your local dns is borked, you will get this error. The fix is to fix your local mx entry. If you have configured an external dns server, then you should go to
DNS functions -> edit dns zone -> email routing for -> check “remote mail exchanger”. That way, we are telling exim to use remote DNS server.
In the backend, it is the process of moving the domain entry from /etc/localdomain to /etc/remotedomain. But don’t edit the file locally because these files are versioned controlled via whm. Change them from Whm as per described above.