Gmail and some other email providers support (and sometimes mandate) that One Click Unsubscribe be implemented in the email system. One Click Unsubscribe is always included in your campaign emails. You cannot disable this option as it is required for bulk email senders. This is all handled behind the scenes and you do not need to configure anything. One Click Unsubscribe tells Gmail to add an Unsubscribe link to the email that you are sending. If the user clicks this link they will be unsubscribed. They do NOT need to visit your website to unsubscribe. The goal is to make the unsubscribe process easy for users.
You can read about this feature below:
https://support.google.com/mail/answer/8151
Once they have unsubscribed, you will see the email in the DNE list:
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