Harald points out some significant issues with pre-existing software. I dispute his conclusion that a failed signature means that the message will be thrown in the trash. Most filters (and certainly any compliant with the criteria being discussed) would quarantine mail with a failed S/MIME signature rather than discard. The second point I would make is that the mailing list software is not immutable. In fact mailing list software is likely to be rapidly upgraded to support anti-spam filters very quickly since the manual anti-spam moderation is a significant burden on the list admin. The problem of broken, obsolete MUAs that do not support S/MIME cannot be allowed to represent a veto. They have had plenty of time to upgrade, there are plenty of free MUAs that work. I have a lot of time for deployment problems when the people with the problem are not technologically sophisticated and the only solutions are geekware. I have no time at all for so-called technologists who refuse to eat their own dog food. That said, yep, non-MIME based signatures may well be necessary. But this is because the authentication model is diferent. We ae not authenticating at the user level, we are authenticating at the domain level. Nobody is going to block individual users of hotmail as spam senders, it is hotmail's job to make sure that their customer's behave. Phill