The fact that sending multiple emails is free or extremely cheap has led to large amounts of junk email, or spam, being sent across the Internet. It's possible to buy lists containing millions of email addresses known to be active, which can be used by companies to send advertisements and marketing information. Spamming is generally frowned upon in the Internet community at large, and it's particularly bad practice to continually spam the same email addresses over and again without giving the recipients a chance to opt out of the list. However, there are many unscrupulous companies that do this; sometimes they will use your request to opt out of the mailshot as verification that your address is “live”. There's no easy way to deal with spam, except to ignore it and delete it as it arrives.