Concept drift is the main problem on spam filtering. Filters are not able to handle information about nuclear problems in Japan and consequently, are not able to accurately filter newest e-mails anouncing potasium iode pills for sale.
Concept drift references a lot of issues that can not be modeled (sometimes are unknown) and can be relevant for solving the target problem (spam filtering). A new spam trend based on advertising a new illegal product is a fact that can not be handle by filters and makes the filtering more difficult. What about new techniques and/or ideas used by spammers to avoid spam filters. Each of then involves changes on target problem and requires a filter update.
We believe that spam filtering can not be solved by using a software product. A spam filtering service is needed to manually handle the effects of concept drift.