Now seems an appropriate time to mention this paper by Dan Bernstein the author of qmail and ezmlm (EZ mailing list manager). ftp://koobera.math.uic.edu/www/docs/mailabuse.html It discusses Mailing list abuse (like what we are seeing now), and types of email abuse. The topics of paper include: False subscription requests Subscription cookie prediction Cross-subscriptions Filter dodging Autoresponder loops Unathorized relaying Unathorized bouncing False unsubscription requests False bounces UCE This section seems most appropriate now: Cross-subscriptions An attacker can subscribe one mailing list to another. Cookies don't help, since every subscriber to the target mailing list---including the attacker's accomplice---receives a copy of the confirmation request. An attacker can subscribe ten mailing lists to each other. This will create a tsunami of mail, destroying all the mailing lists. Advanced loop prevention mechanisms such as Delivered-To don't help, since a message can pass through ten mailing lists in millions of different ways without looping. I propose (1) adding a Mailing-List field to every outgoing confirmation message, (2) adding a Mailing-List field to every distributed message, and (3) refusing to distribute messages that already contain Mailing-List fields. This provides a two-pronged defense to cross-subscription. First, it isn't possible to cross-subscribe lists, since the confirmation message will bounce from the target list. Second, users aren't hurt even if lists are somehow cross-subscribed, since a message distributed from one list will bounce from all the rest. Sublists have to behave a bit differently. Every mailing list has to set the envelope sender on outgoing messages; a sublist checks that it is receiving a message from its parent list's envelope sender. Again the paper is by Dan Bernstein. Dax Kelson Internet Connect, Inc.