I don't disagree, but what are your options when a giant portion of the population uses O365 and outlook.com? How are you going to get them to stop using it, so that legit emails from user-owned servers are not silently rejected?
Most of them wouldn't even care probably...
I use a sending service. I would prefer not having to, but at least all inbound email goes directly to my server.
It depends on the nature of the transaction. If the user needs the email more than you do, you just remind them Outlook deletes your messages, and they'll find a way, and in the long term usage of Outlook will decrease.