The most workable definition of spam in this thread so far was that it is bulk and unsolicited email. This definition is useful on a social level (with humans interpreting both bulk and unsolicited) but becomes less useful when formal definition is needed. (One can introduce variations in messages in ways much more subtle than adding random garbage at the end of the subject line and the body and other minor variations, therefore automatic detection of `bulk' mail is hard.) If we cannot formally define spam (and I do not think we can), we will have trouble devising technical ways of dealing with it. The solution to the spam problem will involve new laws. These need to be supplemented by technical measures. I see two technical measures that would be useful in tackling spam: * Strong mail authentication -- this will allow to deal with mail not coming from strangers; * Ways to require the sender to do a complex computation before the message is accepted -- to deal with mail from strangers. Authentication per se is only useful for building good whitelists. Both individual sender and site-wide authentication would be useful. The second proposal I do not remember seeing in this thread (I am not sure where it comes from). It could work as follows: an SMTP receiver determines whether a message is whitelisted (based on authentication information); if it is not, it uses a heuristic algorithm that determines the probability that it is spam. Further, it assigns a problem for the sender to solve before the message is delivered. The problem must have predictable complexity that the sender must be able to estimate before solving the problem. The sender can then spend the resources to solve the problem to have the message delivered or simply abandon the attempt. A reasonable problem could be, perhaps, to find a string that has MD5 sum that has first n bits equal to a given string. Let me stress that I do not think that the combination of these two technical measures would `solve' the spam problem. But in combination with a legal prohibition of spam in a few countries, it might help. P.S. If people complained more to the ISPs it would help. http://www.internet2.edu/~shalunov/uce/reporting-spam.html -- Stanislav Shalunov http://www.internet2.edu/~shalunov/ Most people can do nothing at all well. -- G. H. Hardy