The List-Unsubscribe header is an optional chunk of text that email publishers and marketers can include in the header portion of the messages they send. Recipients don't see the header itself, they see an Unsubscribe button they can click (outside of the message) if they would like to automatically stop future messages. Gmail is making this suggestion to help prevent user's from opting out by marking the message as spam. Luminate doesn't include a List-Unsubscribe header, but you can make the unsubscribe link bigger and more noticeable to prevent unnecessary spam complaints.