Greylisting – The Next Step In Fighting Spam
What is Greylisting? 
Greylisting is a new method of blocking significant amounts of spam at the mailserver level, but without resorting to heavyweight statistical analysis or other heuristical (and error-prone) approaches. Consequently, implementations are fairly lightweight, and may even decrease network traffic and processor load on your mailserver.
Greylisting relies on the fact that most spam sources do not behave in the same way as “normal” mail systems. Although it is currently very effective by itself, it will perform best when it is used in conjunction with other forms of spam prevention.
The term Greylisting is meant to describe a general method of blocking spam based on the behavior of the sending server, rather than the content of the messages. Greylisting does not refer to any particular implementation of these methods. Consequently, there is no single Greylisting product. Instead, there are many products that incorporate some or all of the methods described here.
For a detailed description of the method, see the following Whitepaper.
