Years past we used to use simple, naive keyword filtering to identify junk email as spam.
But the spammers, driven by profits, got more sophisticated, and software developers came up with fancy Bayesian filtering to solve all that. Clever packages that run at your server to determine spam, massive distributed systems with datacenters to identify patterns, and often worse, paid for, solutions (Norton, Avast, et al) than the problem itself.
Most spammers are
You see, they want to spam, but they also want to appear legitimate. Or at least, just legitimate enough.
Spammers want to offer a way for you to unsubscribe from their crap so they still comply with the law they don’t care about just enough to be able to say “but we gave people a way out!” Each and every spammer, in their cleverness, provides an unsubscribe link.
And everybody knows you shouldn’t click on those unsubscribe links because it will just invite more spam.
But if you add “unsubscribe” to your keyword email filter, 99.9% of all spam suddenly dries up. And if there is a newsletter you legitimately want, it is easy to add to your whitelist.
And so we have come full circle. Back to simple keyword filtering for handling our spam.