
Very good BUT...
1) With end to end encryption you are relying on the capabilities of desktop security software to protect your company and users. Which is the bigger threat? Somebody capturing the email on the network OR somebody with end-end encryption abusing it with NSFW or confidential information?
2) The key advantage of end to end as you describe it here is to protect against internal attackers. Sorry but if somebody is on your network to that extent then you are pretty much stuffed anyway... It doesn't matter how good your end-end encryption is, it won't help against a keystroke logger.