Exchange hybrid: Outlook On-Premise prompts for password when connecting to an Exchange Online mailbox

Outlook 2016, 2019, 365 prompting for a password when adding a second mailbox in Exchange Online, with the primary mailbox still on-premises.

This can happen in these scenarios:

  • A user’s mailbox is on-premises, and they have access to another user or shared mailbox which has already been moved to Exchange Online
  • A remote archive has been created for the user using e.g. Enable-Mailbox -RemoteArchive

In this case, Outlook will prompt for a password when trying to connect to the online mailbox.

Billedresultat for Exchange hybrid on premise users password prompts

Fix it, by adding the following registry key:

  • HKEY_CURRENT_USER\Software\Microsoft\Exchange
  • New, DWORD (32-bit)  Value.
  • Name: AlwaysUseMSOAuthForAutoDiscover
  • Value: 1

This will force Outlook, to always use Microsoft Office 365 Authentication, for Autodiscover, read more here:

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