Expect to lose 1/3 to 1/2 half of your e-mail database every year due to list churn. For many, a significant portion of this churn results from a poorly implemented e-mail address change procedure. Or worse yet, none at all! When you don't help subscribers change addresses, not only will you lose more of them than you need to, but you throw away the money you spent to acquire them, as well as any potential revenue. (E-mail Insider 2/28/08)