The instructions apply to iOS 10 and later.
Go toMail>Accounts.
ChooseAdd Account>Yahoo.

type your Yahoo email address, then chooseNext.
Enter the password for your Yahoo email account, then chooseSign in.
If you’re prompted to enter a code to verify your identity, follow the on-screen steps.

Turn on theMailtoggle switch.
Optionally, enable other items you want to sync, such asContactsorCalendarevents.
Your Yahoo email account will begin syncing with your iPhone.
Your phone requiresSMTPsettings to send mail through your Yahoo account from your phone.
Enter theYahoo Mail SMTP server settingsto send emails.
you might also useIMAP, in which case you’ll use theYahoo Mail IMAP server prefs.
The Yahoo Mail Pro subscription does not contain ads.