I have spent some time searching for something I assumed existed, which is a method of detecting when large quantities of the same message, or messages with nearly the same content (body & subject) are passing through an MTA within a specified time period. It seems to me this could be a useful way to detect not only spam but other types of problems as well - error conditions, mail bombs, etc.
Hmm, DCC works very well for this kind of thing, and you could run it as a local-only server.
