Records that the address unsubscribed from a sender, after you did it in your own browser or in the app (Proxy Mail QA never opens unsubscribe links). From then on, any marketing email from that sender fails after_unsubscribe in check_email and shows in check_unsubscribe, however long after. Name the sender with email_id (the email you unsubscribed from, from count_emails) or service (a name or domain, such as acme.com); with neither, the newest marketing email decides. Pass at for an unsubscribe that happened earlier. Marking twice is harmless. In Labs: works once the person who connected you switches on Marketing and offer emails in Account settings → Labs; until then it answers with an error that says so.
ConnectorOAuth