This feature has actually been available in Gmail for some time, just hidden beneath the spam button.
If you have correctly implemented SPF and DKIM (see
https://community.marketo.com/MarketoArticle?id=kA050000000KyrDCAS) then the e-mail client (Gmail) will display the "unsubscribe" button to the recipient. When the recipient clicks "unsubscribe," the e-mail client will send an automated notification to the mailto address listed in the list-unsubscribe header of the message. This notification informs the sender that the recipient wishes to unsubscribe from future communications. Marketo will process these notifications and mark the lead as unsubscribed as long as you have implemented SPF and DKIM correctly.
A similar feature is also available in Outlook.com (formerly Hotmail).