Not even close - at least not for consumers. See: @aol, @yahoo, @hotmail, @comcast, @gmail etc....
Enterprises usually own their servers, so I would grant that caveat, but even that is going away with cloud providers.
Enterprises usually own their servers, so I would grant that caveat, but even that is going away with cloud providers.
Of course being user owned, by definition users should be allowed to outsource their email service to large companies, and they do, because it's a lot cheaper that way.
But an email server they own themselves, with their own rules, own display preferences, compatible with billions of other email accounts, is well within their reach. And millions of people do just that.
The distinction is that 99.9% of consumers and an increasing number of organizations don't want to do it in house and would rather let someone else do it.
Again, it's not about capability, it's about what people actually are doing.
You couldn't build your own version of fb that can view posts from your friends.
You can setup your own email service to view emails from your friends who are using @gmail.
That's an important distinction.
Try sending a tweet to a Weibo user and see how that works out.
In the name of fighting spam, Google, Yahoo, AOL, etc. have made it a real chore to get messages delivered to their networks. I still haven't figured out how to get messages delivered to AOL.
Usage defines the capability. Just like anything.