When you email your list, you should always include an unsubscribe link so that subscribers can remove themselves from your list. Reputable email service providers will automatically include the unsubscribe link in every email, generally at the bottom of the email. But many marketers feel that even one unsubscribe is a catastrophe and do all they can to stop people from unsubscribing.
In fact, the unsubscribe link is your friend. Letting people who are disinterested off your list makes your email list better. It sounds weird but let me explain.