email showing only on ipad and mac, not iphone

My iphone is suddenly unable to get email of one of my addresses. It keeps asking for a password, which i enter for that account. The mail app then goes through and checks everything, which is fine. But then when I go to email it keeps saying error. I tried turning off the phone. I tried to delete the mailbox to add again, but it won't let me. I've never had this happen and I've had multiple devices for years.