Gmail can do that because they can see into the inboxes of many million of people. So if a mail is flagged as spam somewhere, it can use that info as a signal t

Gmail can do that because they can see into the inboxes of many million of people. So if a mail is flagged as spam somewhere, it can use that info as a signal to classify it as spam in my inbox. That is not the content based plan for spam PG describes. And it comes at a cost. Complete loss of control over your inbox.