GWAVA4 build 112+ SMTP Scanner
What is Greylisting? Answer/
One of the problems using any sort of anti-spam solution is that it is highly reactive. Once a new type of spam message is used there is a little bit of a delay before we can come up with a good way to block it. This is another area where greylisting can be useful. Any time GWAVA sees messages (all messages, or RBL messages) a 4xx level error is returned to the sending SMTP server. A 4xx level error means to try again later. 99% of legitimate email servers will in fact try again later and if they do we will let the message pass by the greylist service. Usually spammers won't try to send the message again--because of this greylisting can give you a little bit of protection against zero-day spam or spam we haven't seen before.
This article was originally published in the GWAVA knowledgebase as article ID 1306.