So you have put the proxy for https://mail.domain.com inside domain.net virtual host. You realize they are different domains right?
You need to set a separate vhost for that subdomain mail.domain.com and put the Proxy there. Or you can set a _default_ vhost as I mentioned before. Default one is the one that is defined first in the list of vhosts no matter if its name starts with 000 or not.
This works. However the main goal is to stop this proxy from hogging all traffic going to 192.168.123.3 which this does not do. To reiterate, when you browse to https://192.168.123.3 using a web browser this produces the webmail login which is not the desired result. I need only https://mail.domain.net traffic proxied to https://192.168.124.3.