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It's not that easy to stop spam ! In fact Google's spam engine is pretty good in this job.
You can't block an IP, neither an address because a spammer is always changing is address and ip obviously.
SPAM filter are based on mathematical statistics. Most of them learn from a bunch of mails (like a thousand of spam mails and a thousand of normal mails) and build arrays of the word present in those mails to finally build a big hash table that says (using mathematical formulas) : "This word in x% of the cases is in a spam mail". When a new mail arrives, it calculate a score which is the probability that the mail is a spam and react accordingly.
Then the important is to train the engine saying "This is a spam" when it didn't recognised one, and the contrary when it put a regular mail in your spam folder.
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