Back in the 1980s, Yosemite National Park was having a serious problem with bears... Make it too complex and people can't get them open to put away their garbage in the first place.
Spam filter developers have a similar problem... spammers learn to send email that's technically clean and has plausible looking content faster and better than legitimate email marketers do.
There is considerable overlap between how spammy email streams appear between the upper quartile of spammers and the lower quartile of legitimate email marketers.
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