Google is trying to make it easier to users to unsubscribe from mass e-mial sent out via Google mail services. For that purpose they are implementing a button, clicking which you get unsubscribed from a marketers' mail.
Google announced the new, rather userfriendly feature during Malware and Mobile Anti-Abuse Working Group conference that took place last week in San Francisco. The new button will automatically appear at the top of a message located right next to name and e-mail address.
Should a recipient be hitting the unsubscribe button, this will trigger a request e-mail to the sender, recommending to exclude the recipient in question from the mailing list. You should keep in mind, that there's just a recommendation being sent out, the unsubscribe button will not automatically remove a mail address from marketers' mailing list.
Source:
Le Comptoir du Hardware