The List-Unsubscribe header is in the unseen header portion of email messages. Recipients don't see the header itself but if the receiving email network leverages the List Unsubscribe Header recipients will see an Unsubscribe button they can trust to unsubscribe from future messages.
The header can look like this to receiving networks:
"The list-unsubscribe has been a very valuable tool for the email ecosystem, from consumers to businesses to mailbox providers. Over the past 20 years, consumers have slowly been trained to mistrust unsubscribe links located in the footers of email and spam, as some spammers would use the unsubscribe link to verify that the email address was a valid, active user. Once the spammers knew that, they would send them even more email rather than opting them out. In some cases, spammers would use the link as a way to install malware on an unsuspecting users’ machine." Microsoft Changes List-Unsubscribe Requirements Melinda Plemel, 1/23/15
Does Marketo implement the List Unsubscribe Header?
Yes, for every email sent from our system Marketo leverages the mailto: List Unsubscribe Header function.
What networks are paying attention to the List Unsubscribe Header?
ISPs and spam filters view it favorably when making filtering decisions because having the List Unsubscribe header can indicate that the sender is actively working to avoid spam complaints. In fact, most major providers like AOL, Hotmail, Gmail, and Yahoo! support List-Unsubscribe functionality.
Gmail supports the List-Unsubscribe functionality and calls it “auto-unsubscribe.” When an email recipient clicks on “Report Spam,” a dialog box will appear that asks if they want to unsubscribe or report the email as spam. If they click unsubscribe, a notification will be delivered to the email address in the List Unsubscribe Header to stop mailing you.