First, my apologies for appearing to repeat a similar query I have just been reading. It is possible the answer is in there, somewhere, but I'm afraid most of it was over my head. So please bear with me.
I have downloaded Thunderbird v.78.2.1 (32 bit) on a brand new desktop running Windows 10. It also has Norton Ultra v. 188.8.131.52. I can receive e-mails, but when I try to send, there is a wait of about 30 secs and then a banner pops up saying "The message could not be sent because the connection to the Outgoing server (SMTP) smtp.virginmedia.com was lost in the middle of the transaction. Try again." Thunderbird on my old laptop running Windows 7 still seems to be working fine.
I have little or no experience of these matters, but I do wonder if the problem could be due to Windows Mail still being on the new PC. The problem with Mail was an ability to send but an inability to receive.
It is unlikely the issue is due to interference from another email client.
Consider enabling logging in Thunderbird to try and identify the cause. To do this:
highlight the following text:
type con > thunderbird-enable-logging.bat
rem Debugging using Thunderbird's logging https://wiki.dovecot.org/Debugging/Thunderbird
start /d "c:\\program files\\mozilla thunderbird" thunderbird.exe
copy the highlighted text by pressingCtrl+C
open theRundialogue box by pressingWindows Key+R
open a Command Shell by typingcmdinto theOpenfield of theRundialogue box and then selectOK
paste the text copied in step 1 into the Command Shell by pressingAlt+Space bar, thenEfollowed byP
press theEnterkey, thenCtrl+Zand theEnterkey to create the filethunderbird-enable-logging.bat
start Thunderbird by typingthunderbird-enable-logging.batinto the Command Shell and then pressing theEnterkey
try to send a message and then quit Thunderbird
in a command shell typefindstr /C:"I/SMTP SMTP Response:" thunderbird*logand then press theEnterkey
Review the resulting output for VM error codes and/or authentication failures
Removethunderbird-enable-logging.batand its logging files by typingdel /p thunderbird-enable-logging.bat thunderbird*log then pressing theEnter key and confirming deletions
exit the Command Shell by typingexitand then pressing theEnterkey
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Thanks for taking the trouble to look at this. Actually, I found a computer whiz nearby and he sorted the problem in about 10 minutes. It cost a few bob, but who cares ? I'm back in business. And he answered quickly and cheerfully several questions on various matters to do with the new PC. Incidentally, and at the risk of sounding like a smart-arse, the Greek word "kudos" is a singular noun, so what I have given you is 1 kudos. There is no such word as kudo. Not your problem, I know, but I could not resist. Since it is highly unlikely that I will be able to help with anyone else's problems, hopefully I shall not be visiting the forum again. However, thanks again.