With greylisting, I find it difficult to tell the actual quantity of spam being sent to the server.
The spam that makes it through greylisting is so obviously spam that nearly all spam not from a Google or Microsoft server goes to the spam folder.
The one exception is some new spammer that lately is sending spam from domains with no SPF record. Because MiaB is a standalone server, it does lack the one feature of having access to a very large pool of addresses for evaluating what users are reporting as spam, which is more powerful than merely the Bayesian filter and a small number of users.
I also find I receive fewer false positives than on Gmail, so I am more confident in assuming that all of the messages in my spam folder are spam.