I am interested in running one server dedicated to email but having the accounts generated from other Virtualmin Pro installations.
Is it possible to move my working Postfix/Dovecot functionality to a separate, dedicated email server?
Then to copy over users accounts under different domains / virtual servers from one Virtualmin box to another, simply backup all virtual servers on one Virtualmin box and then restore to the other Virtualmin box - but restore with only Mail/FTP users and mail aliases box checked. This will create all the mailboxes without the any email in them. If you want email to be restored as well, check the Server’s home directory and web pages box too.
This answers your question about how to move Postfix / Dovecot functionality from one Virtualmin box to another.
It appears you want more. You want to use one Virtualmin box as a control panel but wish to host mail on another - and have the two Virtualmin boxes work in tandem with each other. There are little hacks via which this can be done. Look up the Virtualmin API
You have not shared with us your intent. Why do you want to run two Virtualmin boxes - one for email and one to serve web content? Is it for performance reasons? Yes, web server performance will increase and you will be able to market your services with low TTBF and FCP, as I have done (see Fast web servers - dare to compare TTFB and FCP ) but this cannot be as seamless to the customer as we would like it to be: I haven’t been able to find a practical way in which two Virtualmin servers can be managed via one interface, WRT email users.
Virtualmin is a versatile software and it does offer seamless access to remote databases and database servers via the GUI but not seamless access to remote mail servers, AFAIK.