I just spent about 6 hours on this problem. Since this site had the best answers to the problem (though they didn't work for me), I thought I would post what did work. What I was seeing was that, even after trying the usoft instructions and every possible variation of trying to reconnect outlook 2016 to the server, it kept trying to connect to exchange.activesync versus exchange server, displaying only the email about the changeover. I could go to the Outlook.com website and see all email correctly. I tried wiping the autodiscover entries, wiping both outlook directories under AppData . . . you name it, I tried it, including trying the imap settings. Office 2007 would connect to the correct server no problem. I even recreated the problem on another PC with O365/Office 2016, and corrected the problem by selecting Exchange server as the type of server. However, on this client's PC, nothing would work. What finally did work was running the troubleshooter on the Microsoft Support page. Those things are typically crap, but this one had a specific issue relative to the Outlook.com changeover. I ran that and after some noodling, it corrected the problem and the appropriate email loaded.