Users of the Windows 10 Yahoo Mail app have begun receiving notifications within the app, informing them that the app will cease to work on May 22nd, just a few days from now.
The Windows Store listing for the Yahoo Mail app is still online but the ability to download it has been removed. In addition to this, a large number of users have begun reporting on Twitter that the app has already stopped working for them ahead of the cut-off which suggests that support for it has already begun to cease at least partially.
Generally using the words "email" and "Yahoo" in the same sentence is a bad idea. Do yourself a favour and move to Gmail.