A few weeks ago one of my email sub accounts was locked and to overcome this I was required to change my password via www.bt.com/changepassword since then I have been unable to access my emails via my iPhone even though I altered the password in the account settings I also tried deleting the account on the phone.
After several phone calls to BT and being promised a call from second line support (twice, neither no call happened) I was also told that BT do not support other email clients and I needed to call Apple. I did this and they advised Resetting the phone but this did not work either. I then notice that Outlook wasn’t working even though I knew that I had changed the password details there as well.
I eventually spoke to someone who suggested changing the password from within my BT id (My BT then scroll down to Manage BT email) I did this and I was then able to set up my email on my phone and Outlook without any problems at all. Was it a coincidence? Who knows all I know is it has worked.