I disagree somewhat with this, if it needs to be secure end to end, then the server just needs to support that protocol. The question is, if you don't want to l

I disagree somewhat with this, if it needs to be secure end to end, then the server just needs to support that protocol. The question is, if you don't want to leak any metadata, then how do you put it in the right mailbox? You need a pub/sub type protocol where you can go in and grab the messages meant for you, but the server must not know they're meant for you and you must be able to do so completely anonymously. How do you achieve that? Exchange certainly can't be configured for that, so you need to replace it with an entirely new (and hopefully open source) mail server that supports most of the Exchange functionality - server push etc.