While they do federate, it's a challenge to get them to accept mails from your own mail server. Microsoft's mail servers are notorious for accepting your mails

While they do federate, it's a challenge to get them to accept mails from your own mail server. Microsoft's mail servers are notorious for accepting your mails with a "250 Queued for delivery" and then letting them vanish in a black hole. I've also ran into problems with mail servers of government agencies. So yes, while technically possible, self-hosting your mail server sadly isn't viable anymore for the little guy.