October 1997 - RBL (Realtime Blackhole List) of spam sites appears. It is run by Paul Vixie.
December 1997 - List of 80 million email addresses offered for sale
May 1998 - UUNET (large Internet Service Provider) becomes a major spam source
July 1998 - List of 91 million email addresses offered for sale
Novemeber 1998 - Dotless numeric web sites (e.g. http://3448157907/xx12) are being used to make tracing harder.
June 2000 - Argentina (its TLD .ar) becomes a major spam source
November 2000 - Taiwan (.tw) becomes the spam capital of the world
May 2001 - UUNET finally ceases being a major spam source


References:

• Keith Lynch's Spam Timeline


• The SPAM sketch


• Spamming USENET Thread (1990)


• Brad Templeton's Spam page


• Copy of ARMM spam


• History of a dispute with a spammer


• Copy of HELP ME! Spam


• Usenet's Etiquette-Enforcement Agency