We’re doing a great job spam filtering: that’s why you’re getting so much spam

If 99% of all email is spam, and your ISP blocks 99% of all spam, what
percentage of your email should be spam?

Without thinking, the answer may seem to be 1%, but it’s actually
closer to 50%.


  • For every 10000 emails you receive, 9900 are spam and 100 are
    legitimate (99% of email is spam).
  • Your ISP blocks 99% of the 9900 spams, letting only 1% of the
    spams into your mailbox.
  • One percent of 9900 spams is 99 spams.
  • So, you receive 100 legitimate messages and almost as many spam
    messages, even though your ISP is filtering 99% of all spam!
  • Many people incorrectly think that, if 50% of their email is spam,
    their ISP is only blocking 50% of spam. Not true!
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