Real answer: A duopoly in the email space give two-three companies too much power over SMTP and email in general (and they have no interest in changing it, IE 6 style).
These companies? Microsoft (Exchange, Hotmail/Outlook.com), Google (Gmail, Google Apps, Google Apps for Education), and Yahoo! (Yahoo! Mail).
If these "big three" don't want to support something then it doesn't exist. If they do want to support something then you're adding it to your SMTP/mail server stack regardless of how much you wish to.
Now, I know what you're going to say "what about Sendmail, Postfix, and other SMTP daemons?" But these open source email daemons are very beholden to what the "big three" do. If your SMTP daemon doesn't work with their servers then it is effectively broken and nobody will run it.
The reality is that the SOURCE of almost all non-automated email on the internet comes from these big three (Exchange shouldn't be under-estimated in this context).
So that's why, we're in an IE 6 situation all over again, but this time Google and Yahoo! are also playing gatekeeper in addition to Microsoft.
Real answer: A duopoly in the email space give two-three companies too much power over SMTP and email in general (and they have no interest in changing it, IE 6 style).
These companies? Microsoft (Exchange, Hotmail/Outlook.com), Google (Gmail, Google Apps, Google Apps for Education), and Yahoo! (Yahoo! Mail).
If these "big three" don't want to support something then it doesn't exist. If they do want to support something then you're adding it to your SMTP/mail server stack regardless of how much you wish to.
Now, I know what you're going to say "what about Sendmail, Postfix, and other SMTP daemons?" But these open source email daemons are very beholden to what the "big three" do. If your SMTP daemon doesn't work with their servers then it is effectively broken and nobody will run it.
The reality is that the SOURCE of almost all non-automated email on the internet comes from these big three (Exchange shouldn't be under-estimated in this context).
So that's why, we're in an IE 6 situation all over again, but this time Google and Yahoo! are also playing gatekeeper in addition to Microsoft.