Outlook Error Codes

How to Resolve 401 Unauthorized Errors in Outlook Add-in Debugging

Maira Mehtab Sep 28, 2026 871 views

Question details

The user needs to fix an HTTP 401 Unauthorized error encountered while testing or debugging a custom Outlook add-in.

Product: Microsoft Outlook
Device & OS: not provided
Scenario: Debugging an Outlook add-in within a Microsoft 365 development environment.
Observed behavior: The add-in throws an HTTP 401 Unauthorized response, indicating a failure in authentication, token validation, or account configuration.

Before you start

Ensure you have administrative access to your Microsoft 365 Developer Sandbox and have your add-in's manifest file ready for review.

Solution 1Recommended

Verify Sandbox Credentials and Account Configuration

Ensure you are using the correct test account from your developer sandbox rather than an unrelated personal account.

Authentication conflicts frequently occur when a developer's personal Microsoft account interferes with the Sandbox credentials. Outlook add-ins require specific mailbox identities bound to the active development environment.

Log out of personal accounts

Sign out of any personal Microsoft accounts in your default web browser and the Outlook desktop client to prevent authentication token conflicts.

Sign in with sandbox credentials

Log back into Outlook using the specific administrator or test user account generated by your Microsoft 365 Developer Sandbox.

Verify the assigned mailbox

Navigate to your Microsoft 365 admin center and confirm that the assigned mailbox matches the identity expected by your add-in's authentication flow.

Solution 2

Review Add-in Manifest and Permissions

Check your add-in's manifest file to guarantee that endpoints and mailbox permissions are correctly declared.

Solution 3

Consult Microsoft Q&A Office Development

Post sanitized logs and specific issue details to the official Microsoft developer community for specialized assistance.

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Frequently Asked Questions

Why do I get a 401 Unauthorized error only when debugging my Outlook add-in locally?

This usually happens because the local development server (localhost) is not fully trusted, or the authentication token issued for localhost does not match the allowed domains registered in your Azure Active Directory application or add-in manifest.

Can a mismatch between the manifest and Azure AD cause a 401 error?

Yes. If the Application ID URI or the scopes requested in the Outlook add-in manifest do not exactly match the configuration in your Azure Active Directory app registration, the server will reject the token, resulting in a 401 error.

How do I clear cached credentials in Outlook during add-in development?

You can clear cached credentials by signing out of the Outlook desktop client, clearing your default web browser's cache and cookies, and deleting the contents of the Office cache folder located in your local AppData directory (%LOCALAPPDATA%\Microsoft\Office\16.0\Wef).

Where can I find the exact cause of the 401 Unauthorized error?

Inspect the network traffic using your browser's Developer Tools (F12) if debugging in Outlook on the web, or use a proxy tool like Fiddler for the desktop client. The response body of the 401 error often contains a specific message or sub-status code detailing exactly why the token was rejected.

Microsoft Office Error Codes