Move mail function to another virtualmin server

If I want to plan on doing the following, which guides should I be looking at because I am a little confused..

One virtualmin server for websites, the other virtualmin server for emails.

I can see how MX records will direct email to the email server.. but how do we manage user credentials etc. Which server do I set up the "virtual server (domain)" on? I want the client to actually retrieve their email from the email server so its not going to be a hold and forward.

Any help would be appreciated!

Steven