You have to setup an APP email password on Yahoo, and that place THAT password on both the incoming AND the outgoing server information on the iPhone mail app.
Honestly, it MAY be easier to put the yahoo mail app on the phone and just use the web base username and password in the yahoo mail app, it will set itself up for you.
If not, this may help you -
https://help.yahoo.com/kb/SLN15241.html?guccounter=1&guce_referrer=aHR0cHM6Ly93d3cuZ29vZ2xlLmNvbS8&guce_referrer_sig=AQAAAB93U8mcEjjUghHmi6v5clKl5Ln66CfwjUsLCD4w9fmgoB4t-_fdnu4IrmNi9su8wjy2V_bkvvEPXLDwOu3pEl6o_SnfYMSFFzCpxxVRn3sKDmVRc8Qspu9RxMdvjFdj4AofhNVoeKDx6AM2UrLTDYvSSQxbMJW_EZfJgdcpsQX0
