You're welcome.
How did you create your Yahoo account originally on your iPhone?
Do you have Push access with your Yahoo account?
If so, go to Settings > Mail, Contacts, Calendars.
Select your Yahoo account under the Accounts section, which is the first section.
At the bottom of account page, select Delete Account.
At Settings > Mail, Contacts, Calendars under the Accounts section, select Add Account.
If you currently have Push access for received messages with your Yahoo account, select Yahoo Mail and follow the prompts to re-create your Yahoo account on your iPhone.