If you installed via our install script, you would have gotten MDA configured to use procmail-wrapper. Unless this is an ARM system, in which case, you can’t use procmail (it’s not safe to use it the way we use it without the wrapper).
The default mailbox_command on a Virtualmin system is /usr/bin/procmail-wrapper -o -a $DOMAIN -d $LOGNAME
But, you should figure out what you did to change it, as only root can change it, and so…if you didn’t do it, something or someone else did. Or, your installation didn’t complete successfully (there would have been errors, or it wouldn’t have shown all steps completed…there’s a running tally of steps that have run so it’s easy to know it actually completed).
This, of course, assumes you have /usr/bin/procmail-wrapper on your system. If installed with the install script, then it would be. Again, unless you’re on ARM, in which case, you shouldn’t be doing mail the way we do it, unless you build the procmail-wrapper yourself.
We’ll eventually have repos for ARM, but I haven’t had time to set that up and take on maintaining all the packages and such.