Hi, All of our users are suddenly being prompted to log in when using Outlook 2019.

Error code 50058 indicates an authentication/session issue, and the behavior (desktop Outlook repeatedly prompting, while Outlook on the web works) matches known Outlook client authentication problems.

Based on the available guidance, focus on fixing how Outlook authenticates and clearing any bad cached credentials or legacy/basic auth paths.

Try these steps in order on an affected machine:

1. Clear cached Windows credentials
   1. Open Control Panel → Credential Manager → Windows Credentials.
   2. Remove all entries related to Outlook, Microsoft Office, Microsoft 365, or your Exchange/tenant (for example, entries that contain your mail domain or MicrosoftOffice16 / MicrosoftOffice).
   3. Close Credential Manager.
   4. Restart Outlook and sign in again when prompted.
2. Ensure Outlook is using Modern Authentication (OAuth) Repeated prompts and failures can occur if Outlook is trying to use Basic or legacy RPC-style authentication instead of Modern Authentication. a. Force Outlook to use Modern Authentication for Autodiscover/EWS
   1. Close Outlook.
   2. Open Registry Editor (regedit.exe).
   3. Navigate to: HKEY_CURRENT_USER\Software\Microsoft\Exchange
   4. Create a new DWORD value named: AlwaysUseMSOAuthForAutoDiscover
   5. Set its value to 1.
   6. Close Registry Editor and restart Outlook.
   This forces Outlook to use newer authentication methods (such as Modern Authentication) for web services instead of legacy schemes that can cause repeated prompts.
3. Verify Outlook profile authentication settings If the profile is still using an incompatible logon network security setting, Outlook can continually prompt.
   1. Close Outlook.
   2. Open Control Panel → Mail → Show Profiles.
   3. Select the affected profile → Properties → E‑mail Accounts.
   4. Select the Microsoft Exchange / Microsoft 365 account → Change → More Settings.
   5. On the Security tab, set Logon network security to Anonymous Authentication.
   6. Save changes and restart Outlook.
   For older Outlook builds, this setting being anything other than Anonymous Authentication can cause continuous prompts when connecting to Microsoft 365.
4. Confirm Office can reach Microsoft 365 services If Office is blocked from connecting to the Internet, Outlook cannot complete Modern Authentication and will keep prompting.
   1. In Outlook (or another Office app), go to File → Options → Trust Center → Trust Center Settings → Privacy Options.
   2. Ensure Allow Office to connect to the Internet is selected.
   3. Restart Outlook.
5. If using Outlook Anywhere to on‑premises Exchange plus Microsoft 365 If any users are in a hybrid scenario (Outlook connecting via RPC/Outlook Anywhere to on‑premises Exchange and also to Microsoft 365), Outlook may restrict itself to RPC-compatible authentication and avoid Modern Authentication, causing repeated prompts. In that case, the registry change in step 2 (AlwaysUseMSOAuthForAutoDiscover = 1) is specifically recommended so Outlook uses Modern Authentication for Microsoft 365 connections.
6. If prompts persist for all users Because this affects all users at once, also check centrally:
   • Whether any recent changes were made to authentication policies (for example, disabling Modern Authentication or changing conditional access) that would break Outlook 2019’s auth flow.
   • Whether Basic authentication was recently disabled while some clients or profiles are still trying to use it. Outlook.com and Microsoft 365 are removing Basic authentication in favor of OAuth; any remaining Basic-auth–based configuration will start failing and prompting.
   In such a case, ensure all Outlook 2019 clients are fully updated and using Modern Authentication, and remove/recreate any profiles that were originally configured with Basic authentication.

If, after these steps, Outlook still prompts and fails while Outlook on the web works, collect a fresh error sample (Request Id, Correlation Id, Timestamp) and open a support case so tenant-side authentication logs can be reviewed for conditional access or token issues.

References:

• Outlook prompts for password and doesn't use Modern Authentication to connect to Microsoft 365
• Outlook continually prompts for your password when you try to connect to Microsoft 365
• Password prompt at every Outlook start or can't create profile
• Outlook and other apps are unable to connect to Outlook.com when using Basic authentication
• Do you know how to fix the issue where Outlook keeps prompting for a password even though credentials are saved - Microsoft Q&A
• How to fix the issue with MS Office asking you to log in every time you open an Office app - Microsoft Q&A