Every email campaign you send contains an unsubscribe link. If you don't add one yourself, one will be added automatically. This help document explains how that unsubscribe link works, and what happens to people who have unsubscribed.
How is the unsubscribe link created?
In your HTML page or template, and in the plain text version, there is a special unsubscribe tag. When you send your campaign, each subscriber gets their own individual copy of your email, and that tag is replaced with a unique link for each subscriber.
It is different for each person, and also for each campaign, so you can't just copy an old unsubscribe link and place it into your next email.
What happens when someone clicks unsubscribe?
The unsubscribe link sends the person who clicked it off to our software, where the click is recorded, and the software works out which subscriber, list and campaign it is associated with.
That person is immediately changed from active subscriber to unsubscribed in that list, no need for you to manually do anything at all. In the campaign report, that subscriber will now appear as having unsubscribed.
Suppression list, and unsubscribe settings
By default, unsubscribing from a list will also unsubscribe that address from all other lists in the same account. So if the address is on your 'customers' list and your 'newsletter' list, they will not be an active subscriber in either one.
Unsubscribing also adds that address to the suppression list, which will then prevent you from accidentally importing them into your account after they have unsubscribed.