Have you contacted your email provider on this issue?

No not yet because I saw an archived post with similar symptoms and hoped someone would know what was wrong. By email provider I assume you mean BT and not Mozilla Thunderbird.

Can you change your SMTP settings to a different server?

The only one available is BT Mail

This message seems different, though. Check carefully that there is no link in sig file or that may be suspicious.

Thanks but I don't know what that is or how to check. Is it in account settings?

It might be a signature file in email. For example, some anti-virus programs insert their name in the end of email. If you are using one that does that, such as AVG, disable it and try sending a message. That's just a guess, but there is something there that suddenly changed. In addition to trying that, also create a message and save in draft and then review it to see if there is any additional content in it.

Thanks but my anti-virus program doesn't add anything. I tried saving an email as a draft but on inspection nothing had been added. But on my PC I successfully sent an email from my account using BT Web mail. So that worked but using Mozilla on my PC still gives me the failed to send etc message and using Gmail on my mobile it just stays in the outbox and fails to send.