User Configuration > Administrative Templates > Microsoft Outlook 2016 > Account Settings > Exchange
User
HKCU\software\policies\microsoft\office\16.0\outlook\autodiscover
If set, this policy setting governs the displayed email in auth prompts related to AutoDiscover. Default (0): Prefers the account UPN, when available. If you enable this setting, then AutoDiscover auth prompts will prefer the provided account email (can be either SMTP or UPN depending on which was configured).