Fun fact, Google doesn't play nicely with ESPs either. Most mail providers (Yahoo, Hotmail, etc) use a feedback loop when you report spam. After you mash the

Fun fact, Google doesn't play nicely with ESPs either. Most mail providers (Yahoo, Hotmail, etc) use a feedback loop when you report spam. After you mash the spam button, the ESP that sent the mail is notified that a particular email was flagged as spam. This allows the ESP to curtail spam problems on their end (for example, Mailchimp heavily throttles your emails or outright bans you if your spam rate creeps past a very low percentage). It's an all-around good thing for the ecosystem, with the exception perhaps of publishers that get the unlucky "spam instead of unsubscribe" user action. But Gmail does not participate in this loop. They don't tell any ESP that a user has marked an email as spam...that data all stays in house. Why? Hell if I know - perhaps they don't want to tip off spammers to being detected. On the flipside, reputable ESPs get less leverage on spammers in their network.