Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

Hello Christ1 Your advice was to be just what the doctor ordered. Thunderbird seems to be working fine now. Thank you kindly. Much appreciated.

I got it to work, by combining advice from here and from other pages!

https://support.mozilla.org/en-US/kb/microsoft-oauth-authentication-and-thunderbird-202

This is for my personal Microsoft account (a really old MSN Messenger account I've had since the 1990's and it kept evolving over time).

This isn't for Office 365. This isn't for Hotmail. I would imagine they are similar, though.

First, make sure this works, and you're all set up with Microsoft and their Authenticator app for your account (you need two-factor authentication to be turned on).

https://account.microsoft.com/profile/

Then, go into your email for your Microsoft account, and make sure you have opted into IMAP and SMTP access for your account (it isn't on by default):

https://outlook.live.com/mail/0/

Once the Microsoft stuff is taken care of, go into Thunderbird Tools/Settings, and clean up your old settings:

• Cookies, accept cookies from third-party sites Always (you need this because Microsoft has many sites they use: live.com, microsoftonline.com, and so on)

• Cookies, show cookies, remove anything that looks like Microsoft (clears out stale authentication data so Thunderbird will ask it again and not sit there timing out forever)

• Passwords, saved passwords, remove anything that looks like Microsoft (same)

• General, scroll all the way to the bottom, Config Editor, search for oauth2.issuer and click on any line that looks like Microsoft and hit the trashcan icon to delete those lines

• While you're still in the Config Editor, do the same for oauth2.scope

All this is necessary because Thunderbird has a bug in which it won't bother to clear the old stale information, so it will sit there timing out forever, instead of popping up a new OAuth2 dialog box from Microsoft and allowing you to enter your new information.

Restart Thunderbird.

Now that your settings are correct, go to Thunderbird Tools/Account Settings. Go to your Microsoft account area, and change the settings:

• Server Settings: Server name outlook.office365.com (the old imap-mail.outlook.com won't work anymore)

• Port 993, connection SSL, authentication OAuth2, username is your full email address

• The main settings for your email account (just above Server Settings): Outgoing server, Server name smtp.office365.com (the old smtp-mail.outlook.com won't work anymore)

• Port 587, connection STARTTLS, authentication OAuth2, username is your full email address

Restart Thunderbird again.

It should now work! The Microsoft OAuth2 login should pop up, when Thunderbird accesses your Microsoft account now. Authenticate yourself with Microsoft, using their Authenticator app if necessary.

Send an email from yourself to yourself, so you can verify the full round trip works, both sending and receiving email. It took a few tries but it eventually worked for me!