Has anybody hacked into your email?

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Has anybody hacked into your email? I think mine has been hacked. I had this suspicion when I was trying to register at google. When I wrote my email address in the box where it is asked for, a reply of "the email address has been taken" appeared. I tried it again and the same thing happened. What benefit do this people get by hacking into other people's email? How can we prevent this?