Well, no one wants to read your email right? Wrong! Have you ever heard of Carnivore? That's a system that the FBI uses to harvest all email traffic going through a network. How does this affect you? When the FBI hooks Carnivore up to your ISP's network because they suspect someone of dealing drugs or some other crime, Carnivore will filter through ALL email looking for keywords that relate to the crime in question.
And because the search has to be intentionally vague, you may be sending a request for information about a prescription drug you take and Carnivore will harvest your email to be read by a human agent. Suddenly, the drugs you're taking are now public knowledge. Spammers have also been known to harvest email addresses via captured emails.. Read on over here.