Howdy,
The greylisting is performed by the Postgrey service.
It also sounds like the milter-greylist service was setup on your server – the name there is a bit confusing, as it’s not actually what is performing your greylisting.
The only thing Virtualmin uses milter-greylist for is it’s email rate limiting.
So if you’re experiencing any problems, the key would be to review the Postgrey service to try to troubleshoot the issue.
-Eric