Many users subscribe to websites and lists and later forget that they subscribed, or subscribed unknowingly. Email messages containing anything that looks like an unsubscribe link or instruction may or may not be considered spam by the recipient. To provide users the opportunity to decide, you can quarantine bulk email messages that contain unsubscribe links or instructions, or you can choose to block them all, thereby reducing the load on your mail server. Configure Bulk Email Detection on the Inbound Settings > Anti-Spam/Antivirus page.
To allow all such emails that are not otherwise tagged as spam, set this feature to Off.