since the change over from Yahoo to BTmail haven't been able to send emails from my iphone & to day can't send any from my ipad!!! Having tried some of the advice being posted still getting SMTP wrong address.
I now have had enogh I hate BT also I am getting no end of spam mail even after have marked it as spam & Blocked!!
Make sure all remnants of the previous settings are completely deleted, Apple devices appear to doggedly hang on to incorrect settings.
Make sure you set the accounts up manually rather than letting any wizard complete the configuration.