As spam filters get more advanced, less spam is allowed to enter into user’s inbox so the business model of spammers gets hurt. Instead of thinking that people don’t really like to receive spam and they would prefer less intrusive ways to get publicity, they try to workaround these filters in, sometimes, really clever ways. So, spam filters have to be continually modified and adapted to not fall into these new tricks.

As Bayesian filtering is the most common used technique, this is what spammers try to escape more frequently. We told that Bayesian works by calculating the probability that a word is from spam or from legitimate mail, so what spammers do is modify the messages so they get more probability of being legitimate mail.

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