Earlier quoted context omitted.
You are totally missing the simple fact that the number of blessed email providers to choose from is slowly going down. I've seen ISPs with thousands of clients to give up and move the mailboxes to large players simply because their clients' email was ending up in the spam so often that running the support has gotten too expensive. It's definitely an anticompetitive practice.
It’s just because of spam. All open systems that do not impose a cost to participate are destroyed by spam.
Want to talk about anti-competitive? Gmail will accept mails, provide a 250 SMTP response, then drop the email internally.
That's not right. At all. You can reject the email easily during SMTP exchange, and people have been doing that literally for 20+ years.
No valid excuse here. None. Zero.
And if your 250 OK accept then drop the message, and provide no way to notify, or discuss, or find out why, you make it impossible for a remote admin to fix the problem.
This is 100% on purpose. Yahoo, outlook/hotmail, gmail, collude to resolve issues like this, while blocking all others to resolve issues their own purposefully broken policies cause.
If you see Alphabet with a policy, or action, you can be 100% sure it is aligned to increase market dominance.