A good opt-in process is essential to build your email list. The unsubscribe procedure is also important both for your email sender reputation and customer relationships.
There are many reasons why people may want to leave your list. Not all of those who decided to unsubscribe are dissatisfied or unhappy with your mailings. But this can happen if your unsubscribe process is not clear or doesn’t work. In this case you force the subscriber to hit “This is spam!” button to stop receiving emails from you. Ideally, you should tend to generate as less spam complaints as possible in order not to hurt your deliverability.