To diagnose problems with Thunderbird, try one of the following.

Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

Many thanks. That works and I can send emails now. What now? Is it my virus checker, malware etc. I will ask my IT friend.

Add the TB profile folder as an exception to Bitdefender:

https://www.bitdefender.com/consumer/support/answer/13427/

Help/Troubleshooting, about:profiles, to show the profile location.

Also, disable scanning of secure (SSL) connections in BD.

changed outgoing security to STARTTLS and port 587 and it now works in normal mode. Another question, how do you recover old emails? In the process of fixing the send function an account was deleted then reinstated. All previous emails are gone.

The mail for the deleted account is probably still in the profile folder, in a Mail/<popserver> subfolder (while the reinstated account is in Mail<popserver-1>). Copy the mbox files - the large ones with no extension, named after folders, e.g. Inbox - into Mail/Local Folders. They should appear under Local Folders in the Folder Pane when TB restarts.

Thanks, I think I still have to change the X-Mozilla-Status to 0000. ll good now.