7) Google and Microsoft become dominant in the space and make deliverability harder for everyone who doesn't want to jump hoops to make their server deliverable

7) Google and Microsoft become dominant in the space and make deliverability harder for everyone who doesn't want to jump hoops to make their server deliverable, and even for those who do, sometimes it is literally impossible. For those who want to play the game, they do, but most people move to Gmail and move on.