This is a very strange problem I have encountered. I have a user that is using Office 2016 (just upgraded, problem also existed in previous version, 2013) The user is connecting to an exchange server and is the only user having this issue. Sometimes when outlook starts, the mailbox comes up and works fine. Other times, not all of the emails in the mail box show up, even though they are still there and if logged into OWA they still show up. Sometimes when the user clicks on the inbox, it actually moves up or down in the list of folders in outlook. Sometimes the inbox will not show any emails at all.