What can I do if I get repeated Office 365 password prompts from a Hosted Exchange Mailbox?

Issue:
Repeated password prompts in Outlook when connecting to Office 365 instead of Hosted Exchange.

Background:
This behaviour can occur in environments where a domain is verified in both Office 365 and Hosted Exchange. Outlook uses the Autodiscover service to determine mailbox settings. By default, Outlook may attempt to connect to Office 365 endpoints first, even if the mailbox is hosted on OneNet’s platform. This can result in repeated password prompts, because Outlook is trying to authenticate against Office 365 rather than your Hosted Exchange mailbox.

This is not caused by any configuration change on the OneNet platform, and it is part of how Outlook and Office 365 Autodiscover interact. It has been observed for multiple years and may appear after Microsoft updates or progressive rollouts, which are outside OneNet’s control.

Resolution:
To ensure Outlook connects to the correct Hosted Exchange mailbox, you can add the following registry key:

1. Open the Registry Editor (regedit.exe).

2. Navigate to:

    HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\AutoDiscover\

3. Create a new DWORD (32-bit) Value named:

   ExcludeExplicitO365Endpoint

4. Set the value to:

   1

5. Close the Registry Editor and restart Outlook for the change to take effect.

Notes:

• For Outlook versions older than 2016, the registry path may differ (for example, 15.0 instead of 16.0).

• This change only affects the current user on the machine where it is applied.

• This adjustment prevents Outlook from attempting to authenticate with Office 365 endpoints and should stop repeated password prompts in this scenario.