How to fix Outlook in combination with Hosted Exchange and other MS accounts?

Hi @Hessel Abbink Spaink

Just to clarify, which specific registry value did you change based on Gemini’s suggestion? 

Based on your description, looks like the registry key you modified may be related to disabling Modern Authentication, such as setting EnableADAL = 0, or disabling WAM using DisableAADWAM. 

However, Microsoft has mandatory enforcement of Modern Authentication (token-based secure authentication) for all Microsoft 365 and Hosted Exchange accounts. If outdated guidance was followed and Modern Authentication was disabled via the registry, Outlook will attempt to authenticate using Legacy (Basic) Authentication. Since Exchange no longer accepts this method, the connection is immediately rejected. As a result, Outlook becomes stuck in a password prompt loop, repeatedly requesting credentials for all configured accounts without ever successfully saving them. 

Regarding Apple devices (macOS, iPhone, iPad), these platforms use Apple Keychain as an independent credential management system to authenticate with Microsoft services, which is why the issue may not occur there. 

On the other hand, Windows 11 relies on a system-level authentication framework called WAM (Web Account Manager). WAM manages Work/School accounts and Personal Microsoft accounts (Hotmail, Outlook.com) at the OS level, not just within Outlook. 

If the WAM credential cache becomes corrupted, or if Windows confuses a personal Microsoft account with a work account (PUM / Hosted Exchange), authentication conflicts can occur. In this situation, Windows and Outlook continuously compete for authentication ownership, which causes the password prompt to reappear endlessly without persisting credentials. 

To help us accurately diagnose and resolve the issue, please recheck and confirm exactly which registry key were modified and share the details with us. 

Please understand that our initial reply may not always immediately resolve the issue. However, with your help and more detailed information, we can work together to find a solution. 

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This answer is too abstract/ general for solving the problem I described. Try to be more specific.

The described behavior matches a common issue where Outlook on Windows is trying to connect the same email address to multiple backends (hosted Exchange vs Microsoft 365/Outlook.com) and/or is being redirected to Microsoft 365 endpoints because of previous registry/AutoDiscover changes. The result is repeated password prompts and, in some cases, Outlook using a temporary or wrong mailbox.

Use the following sequence to get Outlook on Windows talking only to the hosted Exchange account again, without reinstalling Office.

1. Fully close Outlook and background processes

• Exit Outlook.
• Open Task Manager and ensure there are no Outlook.exe or MSOSYNC.exe processes running. End them if present.

2. Stop Outlook from talking to Microsoft 365 (AutoDiscover) If registry changes were made that pushed Outlook toward Microsoft 365 endpoints, they need to be corrected so Outlook will resolve AutoDiscover against the hosted Exchange service instead.

From the provided guidance for similar hosted-Exchange vs Microsoft 365 conflicts:

• Set the AutoDiscover-related registry flag so Outlook does not use the Microsoft 365 endpoint as the explicit target.
• This is done under Outlook’s AutoDiscover key (the recommendation is to set the ExcludeExplicitO365Endpoint flag) so Outlook ignores the Microsoft 365 endpoint and uses the hosted Exchange environment instead.
• After adjusting the registry, restart the PC so Outlook starts with a clean configuration.

(If the service provider already added conflicting AutoDiscover/registry entries, they should be reviewed and removed or corrected in line with this approach.)

3. Create a fresh Outlook profile that only contains the hosted Exchange account Because the current profile is now confused by multiple accounts and endpoints, create a new profile rather than reinstalling Office:
4. Open Control Panel → Mail → Show Profiles….
5. Remove the old problematic profile(s) that were using the wrong or temporary mailbox.
6. Select Add…, give the profile a new name.
7. In the Add New Account wizard, enter only the hosted Exchange account:
   • Name
   • Corporate email address (hosted Exchange)
   • Password for the hosted Exchange account
8. Let automatic setup complete. With the corrected AutoDiscover behavior, Outlook should now connect directly to the hosted Exchange server instead of Microsoft 365/Outlook.com.
9. After setup, start Outlook using this new profile.
10. In File → Account Settings → Account Settings…, verify that:
    • Only the intended hosted Exchange account is present (or at least that it is set as Default).
    • No unexpected Outlook.com/Hotmail or onmicrosoft.com accounts are set as default for mail or data files.
11. Re-add personal Microsoft accounts only if needed After confirming that the hosted Exchange mailbox opens correctly and mail and calendars are visible:

• Optionally add the Hotmail/Outlook.com and onmicrosoft.com accounts back in File → Add Account, one at a time.
• Ensure the corporate hosted Exchange account remains the Default account for both mail and data files so professional mail and calendar stay primary.

5. If Outlook still cannot connect to the Exchange mailbox If Outlook still fails to connect to the hosted Exchange mailbox or web services after migration/changes:

• Check AutoDiscover behavior centrally (if in a domain environment) using Group Policy as documented:
  • Import the appropriate Outlook ADMX templates.
  • Under User Configuration → Administrative Templates → Microsoft Outlook → Account Settings → Exchange, configure Disable AutoDiscover and, if needed, Exclude the last known good URL so Outlook does not keep using an outdated or incorrect AutoDiscover URL.
• This prevents Outlook from repeatedly trying a stale Microsoft 365 or wrong endpoint when the mailbox is actually on a hosted Exchange server.

6. Use Microsoft 365 diagnostics if the account is actually in Microsoft 365 If the corporate mailbox has been moved to Microsoft 365 (Exchange Online) and Outlook is failing to connect:

• Run Windows Update to ensure Outlook and Windows are fully up to date.
• Use the Microsoft 365 diagnostic Run Tests: Outlook User Connectivity (requires a Microsoft 365 admin account) to automatically detect and repair common connectivity and profile issues.

7. If there is an overlapping third‑party account using the same address If there is a third‑party provider or another service using the same email address and password as the Microsoft 365 account, Outlook can show errors such as “The connection to Microsoft Exchange is unavailable” when setting up the profile. In that case:

• Either create a new alias (for example, an @onmicrosoft.com alias) and use that for Outlook profile setup, or
• Change the username or password on one of the accounts so that the Microsoft 365 identity is unique.

Following these steps avoids a full Office reinstall and focuses on:

• Correcting AutoDiscover/registry behavior so Outlook targets the correct backend.
• Replacing the corrupted Outlook profile with a clean one that connects only to the intended hosted Exchange mailbox.

References:

• Fix Outlook connection problems for Microsoft 365 account
• Fix Outlook account problems in Office 365 for business
• Outlook doesn't connect or web services don't work after migration to Microsoft 365
• Connection to Microsoft Exchange is unavailable error when a Microsoft 365 user tries to set up Outlook
• Outlook is using a temporary email and I cannot receive emails - Microsoft Q&A