I have tried to send the same emails from Outlook and they also came back as "Address not found". As I said in my first post the email addresses I am using are correct as I have been sending regular emails to them for years.
Both my Outlook account and my Mozilla Thunderbird account are registered under my BT Internet email address.
I did manage to send myself an email from my BT Internet email address on Outlook and it was delivered ok. So it appears to be a problem with sending emails to an external email address.
For some unknown reason the BT Email Upgrade! is affecting all my email accounts not just BT. It was fine up until Thursday 29th February. Perhaps the "Leap Year" has also had a hand in this!!!!!!
I have just sent an email from a gmail.com email address and the same thing has happened. But for some strange reason the "Address not Found" response was sent to my BT Email Inbox and not my Gmail Inbox.
Why has BT email intercepted and processed my Gmail email?
They haven't.
Thunderbird is strange in as much as it doesn't associate SMTP servers with email addresses, it is possible you are sending your Gmail emails via the BT SMTP server.
I am getting very confused and possibly out of my depth.
I just want to be able to send and receive emails as I have been able to do for many years before the changes. I am just a normal email user. Now I can't send emails at all. I have to keep refreshing my BT email in my Browser to get it to work or keep timing out because I have asked it to open a folder or move an email . Or I have to open up new browser tabs to get it to work.
With regards to sending my Gmail email. I did send it from Thunderbird. I have got absolutely no idea how NOT to use the BT SMTP Server.
In Thunderbird click on Tools > Account settings > Outgoing Server (SMTP) . You should see a list of servers. Highlight the the Gmail server and click on Set Default.
Do you have any forwarding rules set up?
One thing is absolutely certain, BT cannot interract with your Gmail account, it has to be something at your end.
As for the prime cause of being unable to send, can you confirm you are either a BT broadband customer or paying for Premium Mail.