When you send an email campaign with unsubscribe link at that time the system will insert an Unsubscribe header option in your emails. When any of your recipients will unsubscribe from email header then you will get an email from Gmail regarding the unsubscribed user's details.

In such cases, you will need to manually take an action to unsubscribe that user from your contacts page so that the user don't receive any further emails from your email account.
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