Greylisting is a new way to combat UCE email typically referred to as "spam.

What Greylisting does is to send a "try again later" response to all incoming emails the first time they arrive.

The SMTP protocol is designed by default to retry sending an email numerous times before giving up.

A properly configured email server will simply send the email again in a short amount of time. The amount of time will vary between email servers.

Since most spammers do not use a properly configured email server or in most cases spoof or fake their IP address they are unable to send the message again.

This means that, for the most part, only legitimate email traffic originating from a properly configured email server will be able to get through as only those servers will send it twice.

The main drawback of this system is that it will delay legitimate email slightly. As stated above the exact amount of time will vary between SMTP servers but is typically around 15 minutes.

There is also a small possibility that a legitimate email server will be configured improperly and not re-send the message. However, in that case the sender would get a bounce message.