Depends on the sender. Using fake opt-out links to build lists of verified addresses is a trick almost as old as spam itself. But services like SendGrid actua

Depends on the sender. Using fake opt-out links to build lists of verified addresses is a trick almost as old as spam itself. But services like SendGrid actually honor unsubscribes, and I believe monitor the unsubscribe rates of their customers to catch those buying lists. I'm not a Gmail user, but doesn't Google click the link in the List-Unsubscribe header for you when you mark as spam? The help article linked from the Google blog post implies they do (or they will): https://support.google.com/mail/answer/81126#subscriptions https://support.google.com/mail/answer/81126#subscriptions