If I ask for an email catalog from The Sharper Image and decide later that I don’t want to get a monthly update from them about their new products, I feel pretty confident that clicking on the “Unsubscribe” button in their emails should do just that. I should be unsubscibed from their email list.
However, if I’m getting emails for things that I don’t want (foreign drugs for example), I would be very wary about clicking on an unsubscribe button in that sort of email. If they’re unscrupulous about sending out mass emails, they’re probably not going to take you off any lists. In fact, this may just be the information they need in order to KNOW that they’ve got a functioning email account to send more unsolicited email to.