The postgrey package included with virtualmin is very old. There have been three releases since the shipping version of 1.34 see https://github.com/schweikert/postgrey/blob/master/Changes they include some fixes to improve upon IP matching related to large pool senders and better IPv6 support.
As well the whitelist in the current version is old and missing many entries needed to avoid long delays from senders using round robin outgoing severs such as office 365, github and facebook. Messages could be delayed by the postgrey delay value * number of servers in their pool resulting in delays of many hours.
Updating the greylist package to the latest version fixes both of these issues. At the very least users should probably be directed update the included whitelist if the package isn't going to be updated.