get rid of your email scanner (antivirus component) When a connection fails you are prompted for a password before the next attempt. If it keeps failing other the issue is antivirus, other times is is incorrect settings. But as you have supplied no information about if the account ever worked in Thunderbird to send and receive mails, I will assume it did and the problem is new.

I see you are on Windows. You can quickly test whether something like AV is causing this.

Start Windows in safe mode with networking enabled - https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

Please post to tell us your results.

Matt, thank you for your answer. Before yesterday I have never had this problem when sending or receiving mails. I don't use another virus scanner other than Windows security virus & threat protection. I have checked all my settings and they all seem to be correct and the same as in the past when there was no problem.

Wayne, thanks to you too. I have done what you requested and in safe mode there was no problem, all worked well. But when I left safe mode the problem still existed.

Because I had a windows 10 update before yesterday, which I installed, someone advised me to undo this update through 'Configure System Restore' and set back a previous restore point and now the problem is gone and everything works as it should.

Matt and Wayne thank you both again for your help.