Outlook 2016 for Mac Export Broken?
Have you tried rebuilding you outlook mac identity? If you haven't, then have a go through that and try exporting your email database again. This is a manual process and may acquire your attention as well as your time. 

Have a look here: https://blogs.technet.microsoft.com/office_for_mac_support_team_blog/2015/01/07/outlook-for-mac-for-office-365-database/

If you are seeking an automatic process then you can have a look here: http://www.mailbackupx.com/how-to-scenerios/how-to-backup-outlook-mac-mails-on-mac.html