Unsubscription links are mandatory for all recipients who do not have an enduring relationship with your business.
All recipients (except level 4 members) will see an "unsubscribe link" in the footer of their email.
Level 4 members don't get the unsubscribe link, as high level members usually have an "enduring relationship" with your business, and you need to ensure you can communicate with these members, without them unsubscribing. For example, staff / coaches / paid memberships. These sorts of relationships need to be ended formally, and not via an unsubscribe link, as you may have other exit administrative actions. It is also to avoid staff etc unsubscribing and then losing their special login privileges without meaning too.
Members can also unsubscribe using the unsubscribe link on your website subscribe page (if you have one).
If a member does not perform any noticable actions within a 2 year period, then they may be automatically reduce to "stale" status.
If a member email address is detected as undeliverable, then the member status will be changed to "email failure" which is the same as unsubscribing them. We try our best to differentiate between temporary mail issues, and explicit "mailbox does not exist" type error messages.
If a customer contacts you and asks to be unsubscribed, you must action the request for them. You cannot ask them to click the link themselves.