I suggest changing the SMTP server to 465-SSL/TLS or 587-STARTTLS and password to normal security

Thank you. I tried changing the SMTP server. Still couldn't send emails. I visited a friend and used their wifi. I could send emails from there. It looks like it isn't Thunderbird. It appears that my ISP has blocked my IP address, for some reason, unrelated to anything that I have done. Will keep trying to get them to sort it out.

My ISP had blocked the sending of email from Thunderbird. It took me an hour on the phone to make them understand that it had nothing to with Thunderbird. Eventually they unblocked me, and I am able to send emails. Problem resolved. For now.....

I still suggest increasing security of the account. Sending to port 25 is not even allowed by many email providers.

Thank you for your advice David. Unfortunately, when I changed the SMTP server to 465-SSL/TLS, or 587-STARTTLS and password to normal security, email wouldn't go. I had to go back to Port 25, which allowed email to be sent. Having weak security may have caused the original problem?

I don't know the original problem, but the virginmedia site and suggestions I found online for blueyonder all stated 465 and SSL/TLS. But you're at least able to send email, and that was the major issue.