The error refers to a timeout. What is the antivirus and its settings for email scanning? It's generally recommended to add the TB profile folder as an exception or exclusion or disable the Mail Shield or similar, depending on the AV product.

If you access the account through webmail, do sent messages appear in the Sent folder?

'avast' antivirus which I disabled but still cannot send 'e'mails via Thunderbird 'E' mails have been appearing in 'sent folder but have not been sent ! What is TB folder please ?

TB profile folder is where all your profile data is stored. In Windows OS by default it is here:

• C:\Users\<Windows user name>\AppData\Roaming\Thunderbird\Profiles\<Profile name>\

If you can stop scanning of the 'Thunderbird' folder and all of it's subfolders and files.

Your Firewall needs to have Thunderbird as an allowed program.

Avast is well known to cause problems:

https://support.mozilla.org/en-US/questions/1312718

When you say the messages appear in Sent but you 'cannot send', does that mean they arrive at your mail server, but aren't arriving at the recipients' end? Are they possibly arriving but being directed to Spam/Bulk, instead of the Inbox?