i done some testing and from what i see enabling the greylisting on postfix from virtualmin force user to install and run the postgreSQL database. But when i look at how greylisting works at least on debian wheezy it seems to use berkerleydb files in /Var/lib/ and not postgreSQL at all. Therefor this dependency seems to be something not used perhaps a missunderstanding because of the name or an ancient requirement that is not anymore used.
Is there a way to remove this dependency for this feature ?