If you want to know how to sign in the Yahoo mail through the Mail app, you should follow the way given here. In case of trouble, you can contact customer care executives.

1. Launch the Yahoo Mail app.
2. Then you will have to enter the Yahoo email address.
3. Now enter the password of your Yahoo Mail.
4. Click to sign in.
5. If you get an error while sign in then call on Yahoo customer service number 1-844-794-2515.

Visit here:

Yahoo customer care number