Because sometimes seeing it says it all :

  • Yes this is my spam folder for the last 18hours (roughly).
  • Yes the red lines are spam that went through my @debian.org address.
  • Yes I'm a DD since a short time, so for most of us, I suspect it's 10 times as bad.

AFAICT, a big part of them directly come from verizon and other ISP address blocks, that a the clever greylisting/rbl use I described would just drop without any effort.