With Azure Rights Management and Azure Information Protection integration into Outlook, Exchange and mobile support, Microsoft is way ahead in terms of ease of use for encrypted emails. In most cases, internal company emails are secured and users don't even know it aside from the little banner at the top. For external sharing it has a fairly easy to use system to share docs or emails and it does federate with external IDs. Company to company emails work without any sort of extra accounts or hassle. Company to end user does have a couple requirements, but it works pretty well. Fully federated experiences are coming to Gmail users too so you won't even have to have a Microsoft ID of any sort if you don't want it. Yes, it works in email clients like Touchdown, and there is an attachment process that works in Apple Mail and such.