All users hosted on Exchange Online as part of Office 365 tenant subscription. Users are synchronized from on-premises Active Directory using Azure AD Connect. AD FS is implemented with Web Application Proxy. Users are using Outlook 2016 external to the corporate network.
After launching the Outlook 2016 desktop client, users are continuously prompted to enter username/password.
Open Registry Editor by running "regedit".
Go to the following hive:
The username/password prompts should cease.