You will need to ensure that the problem is not being caused due to any technical issue on Yahoo Mail’s end. You can check the status of Yahoo Mail online. There are several online services available that can help you know the status of Yahoo Mail. Once you are sure that the problem is not occurring due to Yahoo Mail and still unable to send email from a Yahoo account, the best way to overcome this issue is by uninstalling and reinstalling the Yahoo Mail app. Once you uninstalled the app, you can download the Yahoo Mail app again and check if the issue is resolved.
See also - yahoo mail not getting emails