iPhone mail: "Cannot send mail. The sender address was invalid"
I have just joined the growing list of people running into the above problem. Having read all the messages, I have checked all the known settings and cannnot find anything wrong: The email address is active; I can send emails from webmail; the server settings are all correct and all fields are populated (as per http://bt.custhelp.com/app/answers/detail/a_id/44917/kw/bt%20mail/c/6591)
I deleted the account from my phone and reinstalled it, initially to no avail. Then, weirdly, an email that had been previously stuck was then sent. However, nothing has been sent since.