I'm experiencing something weird. It's an Exchange 2010 environment. I installed Exchange 2016 next to it but none of the DNS names are pointing to 2010 yet.
Still, Outlook clients are suddenly trying to connect over HTTPS (isn't that MAPI/HTTP ?) and giving a password prompt. When you restart Outlook, the password prompt disappears and connection is fine. Sometimes a new profile is required. Hundreds of users are having this problem. What is that? Is 2016 somehow already spreading its influence? :-)