Where has all the pdf spam gone?

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I'm checking our statistics on the amount of pdf spam we're seeing, and after Aug 20 (last week) it seems to have disappeared.

It hasn't disappeared entirely, of course.  But my spam rules that targeted this stuff have gone from a couple million hits per day to a few tens of thousands of hits per day.  There is a very clear delineation at Aug 20.  So, there are a few possibilities:

  1. Spammers have stopped sending pdf spam.  This is either temporary until they start up again (more likely) or they have given up on it completely because the anti-spam community has figured out a way to block it (less likely).
  2. Other spam rules that I have created are hitting pdf spam thus causing some of my tracking rules not to fire.
  3. My stats are wrong and something changed on our internal processes that troll through the logs and update the numbers.
  4. We're blocking the pdf spam mail via blacklists, therefore we don't content scan and hence no statistics.

This is a little weird because we are seeing as much mail as we have ever seen on our network, but my pdf rules have tailed off.