If a customer accidentally clicks on the unsubscribe link within a Matrix email, they will not be able to receive any emails from Matrix going forward. The client has to follow these instructions to opt back in to start receiving Matrix emails again:
Open a new email from the same address where you clicked the unsubscribe link and send a message to firstname.lastname@example.org
You do not have to enter a subject or a message.
The reactivation/opt in usually occurs within a few minutes of sending the email to the optin address.