Sometimes you can receive mail, but can't send. This leads to a lot of confusion. One common cause: your iPhone's joined a WiFi network that is blocking your outgoing mail. Here's the fix.
Quick fix for when you can’t send mail from your iPhone
Turn Your iPhone Sideways: Mail
Mail gets a new look when you turn your iPhone sideways. You might get larger text, and you might get a different layout. Depends on the phone.