Welcome to the new internet. Even when you open e-mails from know individuals, if they visit certain sites or if they open a lot of spam mail, this can get passed onto your computer. Just think of it as one company selling your buyer information to another company, and so on. Mail in that Sweepstakes Entry you got in the Saturday mail and the junk mail will start showing up in your mailbox all the time. Use your "store card" to make purchases and you'll start getting more spam mail from other companies trying to sell you stuff. and if you're not real careful, you'll even start getting spam from the people in your e-mail account's address book. Some of it is perfectly legal and/or harmless, some of it, and probably more then you realize, isn't. I've even got a couple of close friends who I have told that I will no longer open their e-mails due to the places they obviously visit when their online. Just the way it works these days.
