About a year ago I was kindly given a superseded iPhone 5s. With the help of the former owner, I had no trouble unlocking it and moving across to my mobile provider, Talktalk. However I have noticed that I am unable to send or forward e-mails from this phone, which is somewhat of a limitation. The message I always get, in the case of outgoing or forwarded e-mails, is "Unable to send email. A copy has been placed in your outbox. The sender address <my TalkTalk e-mail address> was rejected by the server." Largely owing to this frustration, I have now changed mobile provider to Plusnet and have just tried sending an e-mail using a new gmail e-mail address. I get the same message except that the rejected e-mail address is now the gmail one.
So: what is the server that is rejecting me and how do I resolve the problem please? I am still with Talktalk for broadband but have so far found their tech support less than clueless.
PS The thought occurs to me that, perhaps as an anti-theft device, the phone may have a deep residual memory of its original owner and rejects all attempts by other e-mail addresses to use it but that is a completely uneducated guess. And why are calls and texts fine?