These aren’t Virtualmin questions. This is a general mail protocol question. It’s not about how we do it…it’s what the various tools do, we’re just installing and configuring them with some reasonable defaults (which are, of course, configurable if you don’t like our defaults, but should probably only diverge if you really understand what you’re changing).
The only thing somewhat out of the ordinary in a Virtualmin system (but not that out of the ordinary) is that Procmail is the MDA we use in the standard configuration, which may process the mail with SpamAssassin and/or ClamAV, as well as custom autoresponder and forwarding scripts that we provide. We have a custom user lookup daemon that may run which allows for some per-domain and per-user decisions to be made about how mail is processed (things like whether forwarding/autoresponders are available, spam/AV scanning happens, etc.) which isn’t really possible with a generic Postfix->Procmail configuration, but Procmail is still the MDA. Procmail is likely to change in the future to something that supports Sieve (which is a newer standard for user mail delivery scripting and configuration that handles much of the same things Procmail does), but not in significant ways from a user perspective, except for those users that write their own procmail recipes. The GUI will hide most of the details for most users, as it currently does. Procmail is quite old and has tons of documentation and examples on the web (the only reason we’ll be moving off of it is that it is minimally maintained, so it doesn’t get a lot of developer attention).
The default mail stack in Virtualmin hasn’t changed significantly in the entire life of Virtualmin…it’s not a secret, it’s just boring, and pretty obvious if you look at the relevant config files. I discussed it at length in the distant past when we were settling on what our mail stack would look like (i.e. when we were deciding whether to use sendmail or Postfix, Dovecot or Cyrus, etc.), and it hasn’t changed much since then.
Postfix receives the mail, probably hands it to Procmail which thinks about it and maybe asks SpamAssassin and ClamAV for their opinion about it, Procmail drops it into a mailbox or otherwise acts on it (either forwarding, trashing, etc.), and then the user usually picks it up using IMAP/POP provided by Dovecot.
Also, Jamie doesn’t read the forums. We don’t let him, as he has too much on his plate already. Eric and I try to follow up on tough questions in the forums every couple of days, as time allows. And, if there are bugs under discussion we may escalate them to the ticket tracker.