In AEGEE we have mailing lists. When a mail is sent to a mailing list, it is either distributed, or sent back to the sender. The most common reason for returning mail is, when a sender is not allowed to post to a mailing list. Mails marked as spam and sent to a mailing list, are returned, to the address, that sent them.
Spammers do not put their usual email address as sender, they cheat and use foreigner's email, usually subscribed to their lists of spammed emails. Such messages occassionaly are from a falsified sender, that corresponds to a spamcop-mailbox. Since mails are not allowed to disappear, when such mail comes to an aegee mailing list, it is sent back. As a result, a mail reaches a spamcop-mailbox and the server that sent the mail is marked as spamming-server (blacklisted).
The challenge is to unsubscribe forever aegeeserv from the spamcop blacklists. And grey-listing appears to do exactly this.
This is the base, on which grey-listing was developed. When someone sends an email to a grey-listing enabled domain, initally an error is generated, that affords the sending server to retry later. The receiving server remembers who tried to write whom and from which server. When later the sending server retries to send the mail, or sends another e-mail from the same sender to the same recipient, the message passes trough without delays. Normal servers do retry to resend their mails, spammers usually do not. Thus enableing grey listing for a specific domain delivers less spam to the recipients, but with some delays.
It cannot be said how long delays are caused by the grey-listing, since it is the site-policy specifying how often are deliveries retried. Usually the delays do not exceed to 2 hours.
The idea is to generate for each mailing list a sieve-script telling if a mail is going to be rejected later, and if yes, then the mail will not be even accepted by aegeeserv. The next phase of this plan is to implement a sieve-interpreter (RFC 3028), that applies sieve-scripts to incoming mails. Work is in progress and maybe in Spring 2007 we will enjoy the results. You can contribute by taking over the generation of sieve scripts from mailing lists.