Microsoft add-ins, including ExtendSync for Outlook, run inside an embedded browser environment.
Depending on your Windows build and Outlook version, Microsoft determines which browser engine is used either Microsoft Edge (WebView2) or, in legacy setups, Internet Explorer (IE).
⚠️ 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.
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.
Microsoft Edge (WebView2) Instructions
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 Systems: Internet Explorer Instructions
🕓 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 will retain your authentication and token data, allowing you to stay logged in longer and avoid unnecessary reauthentication.
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.