The recently released Windows 10 preview build 10532 for Windows Insider members now has a new version of the Mail and Calendar app to download. The app, with the version number 6131 (up from 6121), includes a new option to turn its conversation view on or off.
This new feature will likely please users of the Mail and Calendar app who don't care for the conversation view mode, which groups a user's emails into threads rather than by dates. Folks who prefer the old-fashioned way of viewing emails just by their dates will get a chance to change it to this viewpoint by pressing the settings button and then going to the Options menu.
Again, this feature is just for Windows Insiders who have the Windows 10 preview build 10532 installed. There's no word yet on when this will be available for all Windows 10 users. The feature is likely relying on dependencies found only in the latest build of Windows 10 or, Microsoft is testing out the feature before mass deploying.