Why come mails in AEGEE delayed?
Grey-listing in AEGEE.


Behind @aegee.org the greylisting technology is applied in order to prevent the mail server from being blacklisted (marked as spamming server), and to reduce the amount of received spam. However, when a mail comes delayed, there are countless other reasons that could be the cause for the inconvenience. Try to find out where the mail was actually delayed.

The Challenge

From Summer 2006 aegeeserv, the mail server of AEGEE, was regularly blacklisted. As consequence mails sent over aegeeserv were evaluated as very probable spam and endet in spamfolders. Even worse, mails were sometimes rejected and sent back to the sender. All the trouble came from Spamcop: a list of servers sending spam. Spamcop lists servers, which sent mail to any spamcop-mailbox within the last 24 hours. Spamcop tries to recieve as much spam in the spamcop-mailboxes as possible. This is achieved by subscribing spamcop-mailboxes in the lists of the spammers.

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.

What is Grey Lising?

Most spammers send a mail only once. If the mail is not delivered for whatever reason, it is lost.

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 Current Approach

Grey-Listing appears to solve the problem with spamcop-blacklists. So at 18 October 2006 greylising was enabled for all mails coming to @aegee.org, @mail.aegee.org, @lists.aegee.org and @aegee.uni-karlsruhe.de . Due the infrastrucure provided by the University of Karlsruhe it is not possible to enable this technology per recipient, it can be turned on or off for a whole domain.

For the Future

In the future a way to abolish the grey-listing shall be found, as well a second spam-filtering mechanism shall be installed, in order to increase the accuracy of the spam evaluation. The problem is, that with the currently available software it is not possible to say during mail receiption, if the mail will be later rejected by the mailing list software. If this was technically possible, then the mail would have not been accepted to the server, instead of the current approach to bounce inappropriate mails.

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.

Written in April 2007 by Дилян Палаузов