Read your rationale. Sorry, this is extremely naive.

You describe the problems correctly, but your solutions are very shallow.

"Don't store delivered emails"? Great, now we need to do backups of all the individual mail clients!

"Instances that can't be accessed from outside". What happens when such an instance gets compromised? And now _everybody_ on your internal network, who trusts this system without a slightest doubt, will get massively owned.

And on and on it goes...