How to install Virtualmin Full MailServer only?
I’m not sure if Virtualmin provides this option with the install.sh script.
You may, however, do a full install, then remove/disable packages (Bootup and Shutdown) that you won’t need. If you need a browser-based email client, you can either use Usermin or Roundcube. Roundcube does require you to have a web server, either Apache or Nginx. If just using IMAP/POP3, then you can do without them.
Basic packages you need are postfix, dovecot, spamassassin (and clamd) for spam and malware checking. And MySQL for roundcube.