You may be losing people who unsubscribe from your email list who don’t really want to.
They may like your messages, but you’re sending the wrong stuff or mailing too often.
Ask unsubscribers if they want to get off the list completely, fewer emails, or different content. See this great example from Nordstrom (click for a close-up).
Important: Always make sure that they can still unsubscribe simply and clearly, with no tricks or obstacles. Nothing will burn a customer faster than making it hard to get off a list.