They can fix it. They have certainly figured out how. But their "killer feature" is not that you don't receive spam, it's that the mail you send isn't flagged as spam by their fellow oligopolists.

We're now at the place where it's virtually impossible to run your own mailserver and have the mail delivered, consistently at Gmail and Outlook/Live/Hotmail. At least not without hours a month tuning, re-configuring, monitoring etc.

Basically, Gmail, Apple Mail, Microsoft, Yahoo (and to lesser extent, Fast-email, proton, or one of the handfull of dedicated email providers) have cemented an oligopoly. You must invest serious infrastructure, time and effort, or else your mail will be /dev/nulled (at random, often).

This "anti-spam" works, reasonably well. Because Gmail can now trust that Microsoft has measures in place to disencourage new accounts from sending large amounts of mails - and vice versa. Obviously Gmail can trust other Gmail accounts. And so they have a win-win-win.

win: No need for heavy, resource-intensive spam-training or scanning for the bulk of incoming mail - if its from a fellow BigTech, let it through. Win: an almost impossible high barrier to entry for any serious competitors. Win: Lock in, because anyone wishing to move will see their email not reach the inboxes of users at other Big Tech - aka the vast majority of inboxes.