They do in a way, the 'List-Unsubscribe' header: https://support.google.com/mail/bin/answer.py?hl=en&answer=81126 https://support.google.com/mail/bin/answer.py?

They do in a way, the 'List-Unsubscribe' header: https://support.google.com/mail/bin/answer.py?hl=en&answer=81126 https://support.google.com/mail/bin/answer.py?hl=en&answ... Appears to work with Outlook.com/Hotmail, and Gmail. Not sure about Yahoo, but those two alone cover a pretty big group.