I was with a customer in Bradford on Avon installing their new iMac and they happened to mention to me that they couldn’t send email through their iPhone. They were on Orange. Orange tech support had failed to resolve it, as did the “Genius” in the Bath AppleStore.
To keep it brief, make sure you have SSL off, SMTP Authentication off, and use port 25 in mail account Advanced settings.
If you need to send emails when connected via non-Orange networks, all bets are off and you’ll have the mess around with trying to authenticate to their smtp server. If you use your our mail server, try using a non-standard port like 26.