Iolo – I had exactly the same thing a few weeks ago. You’re not with BT by any chance? I contacted ISP (BT) and they talked me through deleting the account then setting up email account from new. It initially didn’t work but when we went back into SMTP settings in the email account, there were a few in there that needed to be unticked and ticking a few others. This sorted it. I’m not technical and no idea exactly how or why this worked but there may be enough info there for you, or someone else, to sort it. Interestingly, other email accounts with the same ISP in the household had no such problems on Apple devices, only me. There has apparently been an issue with some accounts with Yahoo getting hacked and someone from BT told me they’re in the process of changing people over from Yahoo mail – something like that. Not sure if that’s anything to do with it.