ExtendSync for Outlook runs inside an embedded browser environment provided by Microsoft.

Depending on your Windows build and Outlook version, this browser is either:

Microsoft Edge WebView2 (modern and supported), or
Internet Explorer (IE) (legacy environments only)

⚠️ Important:

Internet Explorer has been retired and is no longer supported as a standalone browser or rendering engine for Outlook add-ins in modern Microsoft 365 environments.
ExtendSync (and all current Office add-ins) now run using Microsoft Edge WebView2.

👉 Learn which browser engine your Outlook add-ins use

Why This Matters

If the browser used by ExtendSync (Edge WebView2 or IE in legacy systems) is configured to clear cache on exit, your login session and token data will be deleted every time you close Outlook.

This will cause you to log in more frequently.

To prevent this, make sure cache clearing is disabled in the browser engine that Outlook uses.

Recommended: Microsoft Edge (WebView2)

Most modern Outlook installations use Microsoft Edge WebView2.

Open Microsoft Edge.
Go to Settings → Privacy, Search, and Services.
Scroll down to Clear browsing data on exit.
Click Choose what to clear every time you close the browser.
Make sure Cached images and files is turned OFF.

💡 Tip: Keeping cached data allows ExtendSync to remember your session, preventing repeated authentication prompts.

Legacy Only: Internet Explorer (If Still Used)

🕓 Note:
These steps apply only to organizations still running legacy Outlook builds or Windows environments that use IE as the embedded add-in browser.

Open Internet Explorer.
Go to Settings → Internet Options.
Under the General tab, find the Browsing History section.
Ensure Delete browsing history on exit is unchecked.
Click Apply, then OK.

Result

Once cache clearing is disabled:

ExtendSync retains authentication and token data
Users stay logged in longer
Repeated login prompts are avoided

This significantly improves the day-to-day experience when using ExtendSync for Outlook.

Why Internet Explorer Is Still Mentioned

While Internet Explorer 11 was officially retired on June 15, 2022, some older or on-premises environments may still use it (or the old EdgeHTML engine) as the fallback for Outlook add-ins.

In these cases, the IE instructions above remain relevant.

For all modern Microsoft 365 and Outlook versions, ExtendSync uses Microsoft Edge WebView2, which fully supports modern authentication and cache management.