1. Tap Settings on the iPhone home screen.
2. Scroll down to Accounts & Passwords and tap it.
3. Select Add Account.
4. Tap the Yahoo logo on the screen that opens.
5. Enter your full Yahoo email address in the field provided and tap Next.
6. Enter your Yahoo Mail password in the field provided and tap Sign In.
7. Confirm the indicator next to Mail is in the On position. If not, tap it to activate it. Slide the indicators next to Contacts, Calendars, Reminders, or Notes to the On position if you want them to appear on your iPhone.
8. Tap Save.

In iOS 10:

1. Tap Settings on the iPhone home screen.
2. Go to Mail.
3. TapAccounts.
4. Tap Add Account.
5. Choose Yahoo.
6. Tap your name under Name.
7. Type your full Yahoo Mail address under Address.
8. Enter your Yahoo Mail password under Password.
9. Tap Next.
10. You will see choices for accessing Mail, Contacts, Calendars, Reminders, and Notes for this Yahoo account. Slide the indicator to green for On for each that you want to access on the iPhone.
11. Make sure Mail is On to receive email in iPhone Mail.
12. Tap Save on the upper bar.

Now the account should appear in the Mail app Accounts list.

Mail App Options for iPhone
You can change your options for this account in the Settings > Accounts & Passwords menu in iOS 11 (Settings > Mail > Accounts in iOS 10 and earlier). Tap the arrow at the far right of the Yahoo account, and you can toggle whether or not to access Mail, Contacts, Calendars, Reminders, or Notes. This is also the screen where you can select to delete the account from your iOS Mail app.

Next, to the Account name at the top, tap the arrow on the far right to see the name and email address associated with the account. You can change the description of the account or change the outgoing SMTP server settings, although these are usually configured automatically.
You can also access the Advanced settings to set mailbox behaviors, and indicate where to move discarded messages and how often to remove the deleted messages.

If you have any problem with sending outgoing mail, check the SMTP server settings. While these should pass seamlessly from Yahoo to iPhone mail, incorrect SMTP settings could be the source of the problem.

I hope this helps.

Regards,
Lokesh