Outlook online will log you out

Configure the normal browser profile to retain and permit Microsoft sign-in cookies, then disable privacy or content-blocking extensions for Outlook; private browsing working while the normal profile fails points to browser-profile handling rather than Outlook intentionally ending every session.

Fix the normal browser profile

Since clearing all cookies has only helped temporarily and private browsing already works, use these more targeted checks:

1. Allow cookies for Outlook and Microsoft authentication
   • Ensure the browser is not blocking cookies or automatically deleting them when Outlook closes.
   • If third-party/cross-site cookies are blocked globally, add site exceptions for:
     • outlook.office.com
     • login.microsoftonline.com
     • microsoftonline.com
   • In Microsoft Edge, set Settings → Privacy, search, and services → Tracking prevention to Balanced for testing.
   Outlook uses cookies across its own and Microsoft authentication domains to preserve authentication state. For example, ESTSAUTH maintains the current browser session, while ESTSAUTHPERSISTENT supports remaining signed in after closing the browser.^1^
2. Disable extensions selectively
   • Temporarily disable ad blockers, tracker blockers, script blockers, cookie managers, and security/privacy extensions.
   • Close all browser windows, reopen the browser, and test Outlook.
   • If that works, re-enable extensions individually to identify the conflicting one.
   Microsoft specifically notes that Outlook can have problems when AdBlock, UBlock, or pop-up blockers are enabled.^2^
3. Test a new browser profile
   • Create a new profile in the same browser rather than another private window.
   • Sign in with only the affected Microsoft 365 account and open Outlook.
   • If the session survives, migrate bookmarks to the new profile instead of repeatedly clearing the old profile’s cookies.
4. Use a current supported browser
   • Test the latest Edge, Chrome, or Firefox on Windows; on macOS, use current Edge, Chrome, Firefox, or Safari 16 or later.^2^

Distinguish a browser problem from an organization policy

A tenant idle-session policy is more likely when Outlook works normally at first and later displays “Your session is about to expire” after inactivity. That policy applies per browser session and can sign the user out of all Microsoft 365 tabs in that browser.^3^

For a work or school account, ask the Microsoft 365 administrator to check:

1. Microsoft 365 admin center → Settings → Org settings
2. Open Security & Privacy → Idle session timeout.
3. Review whether it is enabled and the configured duration.
4. Modify or disable it only if that matches the organization’s security requirements.

This setting is tenant-wide and cannot be scoped directly to individual users or groups. Microsoft Entra Conditional Access is required when different session behavior is needed for selected users or device conditions.^3^

The older Outlook-specific activity-based authentication timeout began retirement in January 2024, so administrators should prefer the Microsoft 365 idle-session setting for current configuration.^4^

If a new browser profile with permitted cookies and no extensions still signs out immediately—and no tenant timeout prompt appears—the next step is Microsoft 365 support, because the remaining issue may involve the account or tenant session rather than local browser state.

References

1. Cookies used in Microsoft 365 web apps and services
2. Supported browsers for Outlook on the web and Outlook.com
3. Idle session timeout for Microsoft 365
4. Activity-based authentication timeout for Outlook on the web in Office 365