A Microsoft application for managing email, calendars, contacts, and tasks across devices and platforms.
I moved your question to the Insider forum, as you are using an Insider Fast build of Outlook for Mac.
There is a new build (170712) of Outlook available to Insiders. In Outlook, go to the Help menu and choose Check for Updates to launch AutoUpdate, which should install the new build for you.
If you still have the problem after installing the new build, you might try renaming or deleting NormalEmail.dotm. To do this, you must ensure that you File > Quit both Word and Outlook. Then go to the hidden user Library folder.
How to Access Hidden User Library folder on Mac OS ...
The file path is
After you delete or rename the NormalEmail.dotm, the next time you open Outlook it should make a new, pristine copy.