I'm not sure yet what it shows, but clearly something about the nature of the spam reaching CSAIL's mail exchanger changed a couple of weeks ago:
Notice how the number of messages rejected outright (red line) usually tracks the number of received and completed messages fairly closely. Then things change fairly dramatically. I still don't know how to explain the increase in rejected spam, especially considering that there's no apparent increase in mail that actually gets processed. It's as though we're somehow rejecting an entire spam campaign, but that doesn't seem very likely unless the spammers are really clueless.
There are several reasons why a given message could be rejected by our servers. I haven't yet investigated deep enough to see which mechanism is responsible for this increase.
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