Another way for spammers to get your address is to legitamately purchase it. They buy a list which was purchased from a site who purchased it from a site who bought it from Yahoo or Hotmail.
After all, one of the biggest assets for sites like Yahoo, MSN, and Hotmail is their list of active, real e-mail addresses. If they can get even 5 cents per e-mail, they could probably get $200,000 or more... which always makes the accounting people very happy.

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