Yes, Virtualmin offers mail.domain.tld for incoming and outgoing server for mail clients which connect to virtual servers so if you are transferring a domain from one Virtualmin server to another Virtualmin server, the settings remain the same.
Outlook or other email client will not have to be reconfigured to work on the new server. If you restore the virtual server with the passwords intact (Virtualmin’s backup will do this, of course) and if you can change the IP / DNS quickly enough (by lowering TTL) the shift might be so seamless that your clients might not even notice that you have shifted them to a new server.
If you’re proxying through Cloudflare, you need to make sure your mail-related names are not proxied. Cloudflare does not proxy mail protocols (they have a mail forwarding service, if you’re paying for it, though).