iPhone 5S, ios 9.3.5, recently I find I can receive e-mails on my btinternet account, but cannot send. I have tried resetting login and password, deleting the account and adding it back in and restarting the iPhone. None of these seem to have helped. Other mail accounts such as gmail seem to work OK. The same mail account works fine on the iMac. Any suggestions as to other things I should try before I go down the - gruesome - route of the BT helpline?