Basically, for the first time when receiving an e-mail with a given
sending IP address, sender and recepient, the server will temporarily
fail delivery, asking the sending server to retry.
Most spam software doesn't retry, so most of that spam will be blocked.
There are other ways to compensate for greylisting, but that will then
make the spammer's efforts more detectable by other methods.
So not a final solution, but perhaps another good tool for the toolbox.