There is a possibility that you don’t enable cellular data. So when your iPhone is out of WiFi, your Yahoo emails cannot be received. Go to Settings > Cellular > Toggle Mail App on. Set SMTP server. To solve yahoo email problems iPhone, Go to Settings > Accounts & Passwords > Yahoo! > Account > Outgoing Mail server SMTP > Other SMTP Servers. In Other SMTP Servers Host name is smtp.mail.yahoo.comO > Enter full Username and Password > Set Port to 465, and SSL to Yes. Enable “Allow apps that use less secure sign-in”. Launch Yahoo and sign in > At the top of the page tap on Your name > Account info > Account Security > Scroll down to find “Allow apps that use less secure sign-in”, and enable it.