I've found something really strange. Most of the time Thunderbird works great (why I use it) but after some time, sometimes hours, sometime weeks, I start getting authentication errors. Closing and restarting Thunderbird doesn't help, HOWEVER if I restart the PC then it does start working again for several days. I have two other *.outlook.com account and these do not appear to suffer from these issues, I can send emails from these even when my primary account reports "Authentication errors", particulary for the SMTP server. I've deleted the account in the Settings ¦ Outgoing Server (SMTP) list, then recreated the SMTP details. This DOESN'T make Thunderbird work when send is reporting authentication errors, however a restart immediately clears the problem. Has anyone any ideas, I use the portable version of Thunderbird (for years now) but the problem has only really started in the last year

Have you tried changing your antivirus? Like perhaps using the windows defender product that comes with Windows..

I've used Defender for years as it seems as good as the other AV solutions. IF it was an AV issue surely my other 2 *.outlook.com accounts would suffer similar issues? What I find really weird is that it works OK for days then just stops, almost as though something glitched in TB, which a full PC restart resets. I've asked before, but had no answer, is there any way of enabling a logging of the authentication messages ?