The timetoreply extension works in Outlook on Windows, Mac and the web. It doesn’t run in the Outlook mobile apps for iOS or Android.
In Outlook, the extension runs inside a small built-in browser. Which one Outlook uses depends on where you use Outlook:
| Where you use Outlook | Built-in browser |
|---|---|
| Outlook on Windows | Microsoft Edge WebView2 |
| Outlook for Mac | Safari |
| Outlook on the web and Outlook.com | The browser you open Outlook in |
The extension works in Outlook 2019 and later. It doesn’t work in Outlook 2016 or earlier, so update Office or use Outlook on the web instead.
For the full details, see Microsoft’s guide: Browsers and webview controls used by Office Add-ins.