Does virtualmin use postfix or sendmail? I'm trying to buy a book on it, but I'm not sure what it uses!
Virtualmin Professional uses Postfix in the default installation. Sendmail is also supported by the software, but isn't auto-configured for you during installation the way Postfix is.
We have a recommended reading section that includes some know-to-be good books on the relevant components of a Virtualmin system, including a pretty good Postfix book:
Check out the forum guidelines!