File Cannot Open

Fix Excel Keeps Requesting Sign-In for Outlook Spreadsheets

Maira Mehtab Sep 21, 2026 869 views

Question details

Users are unable to open Excel spreadsheets directly from Outlook because the application gets stuck in a continuous authentication loop.

Product: Microsoft Excel, Microsoft Outlook
Device & OS: not provided
Scenario: Opening a spreadsheet attachment directly from an Outlook email client.
Observed behavior: Excel repeatedly prompts the user to sign in, followed by a sign-in failure message, preventing the file from opening on the computer. However, the same file opens successfully on a mobile device.

Before you start

Ensure you have access to a local folder on your computer (like the Desktop or Downloads folder) to save the attachment temporarily.

Solution 1Recommended

Save and Rename the Outlook Attachment Locally

Bypass the Outlook-Excel authentication glitch by saving the file to your computer's local drive and giving it a new name before opening it.

When opening attachments directly from Outlook, Excel opens the file in Protected View from a secure temporary folder. This process can sometimes trigger an authentication bug with Microsoft 365 servers, causing a sign-in loop. Saving the file locally and renaming it severs this temporary link, allowing Excel to treat it as a standard local file.

Save the attachment to your computer

In your Outlook email, right-click the Excel spreadsheet attachment and select 'Save As' from the context menu. Choose a local destination such as your Desktop or Documents folder.

Rename the saved file

Navigate to the folder where you saved the spreadsheet. Right-click the file, select 'Rename', and type a slightly different name for the file (for example, add '-local' to the end of the original filename) and press Enter.

Open the file in Excel

Double-click the newly renamed file to open it. Excel should now load the spreadsheet normally without triggering the sign-in prompt.

Workaround Successful: By renaming the file, you bypass the cached credentials and temporary file path that were causing Excel's security protocols to malfunction.

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Frequently Asked Questions

Why does Excel keep asking me to sign in when opening an Outlook attachment?

This happens due to a synchronization glitch between Microsoft Outlook and Excel's Protected View feature. The Microsoft 365 system attempts to verify your cloud license or permissions for the temporary email file, resulting in an endless sign-in loop if the authentication server fails to validate the request.

Can I disable the sign-in prompt for all downloaded attachments?

While you cannot completely bypass account authentication for Microsoft 365, you can reduce file-blocking prompts by opening Excel, navigating to File > Options > Trust Center > Trust Center Settings > Protected View, and unchecking 'Enable Protected View for Outlook attachments'. Note that disabling this removes a layer of security against malicious files.

Will signing out and back into my Microsoft account fix this issue?

Sometimes. If the local renaming trick is too tedious, you can try opening Excel directly (not from an attachment), navigating to File > Account, signing out of all Microsoft accounts, restarting your computer, and then signing back in. This refreshes your credential tokens.

Why does the same Excel file open on my mobile phone but not my computer?

Mobile versions of Excel handle temporary email downloads and cached credentials differently than the desktop application. The mobile app bypasses the specific desktop-level Protected View and temporary folder protocols that trigger the desktop sign-in failure.

Document & File Issues