Sometimes the Preferences in Outlook get corrupted, leading to all sorts of troubles (user interface, messages not sending, app freezing, etc).
The Preferences file hosts a number of settings, but nothing critical per se as the accounts and other related important parameters of the app are saved int he Identity itself instead of the Preference file.
The best way to delete the preferences in Outlook is to quit Outlook, launch the Terminal and use the command:
defaults delete com.microsoft.Outlook; killall csprefsd
The first half of the command clears the preferences while the second half purges the cached parameters.
After that you can relaunch Outlook and check whatever Preferences (Outlook > Preferences) you might want to re-set.