I finally figured out how the e-mail works on my mobile phone.
I could always receive messages on my phone but I could never reply or send.
It was an off the cuff remark on a forum post that triggered the lightbulb moment. I can only send via my ISP’s mail server if I am connected via my ISP! Obvious with hind sight !
All I have to do is send all my messages via the Orange mail server.
Works a treat!