Why has e-mail taken the world by storm?

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Across
  1. 3. an extreme weather event such as a typhoon
  2. 4. interaction with another person such as speaking
  3. 6. joined with another person for life
  4. 7. to erase
Down
  1. 1. to burst or to break apart
  2. 2. a person you do not know
  3. 5. a single person