Outlook not working

Because classic Outlook is failing for everyone while Outlook on the web works, a Microsoft 365 administrator should first check the tenant’s Exchange Online service health, keep users working in the web client, and then test desktop connectivity on one affected computer.

1. Check for a tenant-wide incident

1. Sign in to the Microsoft 365 admin center with an administrator account.
2. Go to Health → Service health.
3. Review Exchange Online and Microsoft 365 Apps for active incidents or advisories.
4. Open any relevant issue and compare its User impact and start time with your outage.
5. If nothing is listed, select Report an issue from the Service health page.

Microsoft recommends checking service health when multiple users report Outlook problems. Service Support and Helpdesk administrators can also view this page.^1^

Until desktop access is restored, users can continue through Outlook on the web, since you confirmed that mailbox access there is working.

2. Run the administrator connectivity diagnostic

If Exchange Online shows healthy:

1. Select Run Tests: Outlook User Connectivity from Microsoft’s Outlook connection troubleshooting page.
2. Sign in with a Microsoft 365 administrator account.
3. Test one representative affected user and computer.
4. Record the result, error text, timestamp, and correlation ID, if shown.

The diagnostic performs automated connectivity checks and suggests applicable corrections. It is unavailable for Microsoft 365 Government, Microsoft 365 operated by 21Vianet, and Microsoft 365 Germany.^2^

Also capture from the same computer:

• What happens when Outlook starts: failure to open, password loop, Disconnected, Trying to connect, or another message.
• The full Outlook version from File → Office Account → About Outlook.
• Whether the failure occurs on a different Windows network.
• Whether recently deployed security, proxy, firewall, authentication, Office, or Windows changes coincide with the outage.

Do not make registry or organization-wide profile changes without a matching error or diagnostic result.

3. Isolate the desktop profile on one pilot computer

If the diagnostic points to a profile problem—or does not identify a tenant/service issue—test a new Outlook profile for one user only:

1. Exit Outlook.
2. Open Control Panel → Mail → Show Profiles.
3. Select Add, enter a profile name, and complete the account wizard.
4. Select Prompt for a profile to be used temporarily.
5. Start Outlook with the new profile.

Out-of-date software and corrupted profiles are common desktop connection causes, but an organization-wide outage should not be treated as a profile issue until a pilot test demonstrates that this resolves it.^2^

If the new profile works, deploy the correction in a controlled manner. If it also fails, retain the diagnostic output and escalate rather than recreating every user’s profile.

4. Open an urgent Microsoft 365 business support case

A Microsoft 365 administrator can create a request directly:

1. Open the Microsoft 365 admin center.
2. Select Help & support at the bottom-right.
3. Describe the issue as:
   “Organization-wide classic Outlook for Windows access failure; all users affected; Outlook on the web works.”
4. If suggested results do not resolve it, select the Contact Support headset icon.
5. Choose phone or email, confirm the contact information, and select Contact me.^3^

Include:

• Number and percentage of affected users.
• Outage start time and time zone.
• Windows and Outlook versions.
• Exact errors, screenshots, timestamps, and correlation IDs.
• Service-health status and incident ID, if applicable.
• Connectivity diagnostic results.
• Confirmation that Outlook on the web works.

If every subscription was purchased through a Microsoft partner, contact that partner; direct Microsoft support access requires at least one subscription purchased through Microsoft.^3^

Remove the personal phone number from this public post. Provide contact information only inside the authenticated Microsoft 365 support request.

References

1. How to check Microsoft 365 service health
2. Fix Outlook connection problems for Microsoft 365 account
3. Get support for Microsoft 365 for business