You mentioned MySQL in the topic, so I will assume you are using that.  MySQL uses network based connections naturally.  If you are working on the machine that hosts the server, the login becomes user@localhost.  To login from a remote location you will need to create a user account for user@remotehost.  If you have a LAN domain, the remote host will probably be the fully qualified domain name of the host.  If are accessing remotely, from the internet, you will need to add port forwarding in addition to the user@host.

You can also use wild cards to mask out all or part of the host name.  This degrades the security so you will need to think carefully about this.
 

Thanks for the reply.

The two servers are on a LAN domain, and the mysql server has an appropriate user account.

The email server is already accessed successfully by an older qmail server that works, but needs to be upgraded. I have no idea how this older server was set up to read from the remote mysql- all I know is that is works.

The part I am stuck on, is directing a new test version of qmail (and vpopmail specifically) to connect to the remote  mysql server instead of its local mysql.

Can this be achieved through wildcards?

In my 3rd paragraph I meant 'Mysql server', not 'email server'..

The 'mysql' server is already accessed successfully by an older qmail server that works, but needs to be upgraded. I have no idea how this older server was set up to read from the remote mysql- all I know is that is works.