Use the correct server names: Incoming server: outlook.office365.com Outgoing Server: smtp.office365.com

Also see https://support.mozilla.org/en-US/kb/microsoft-oauth-authentication-and-thunderbird-202#w_changes-to-authentication

On top of that, Norton may also interfere with the authentication process. Try to start Windows 10 in safe mode with networking enabled. Does the problem go away?

Windows safe mode disables anti-virus software. This is a troubleshooting step, not a solution.

Thank you very much for reply and help. Yes, I have used the correct names for servers in trying to make this work ...I just used a shorthand to keep my request for help readable. And yes, I have been reding and trying to follow the Thunderbird post you cite for several months...I have tried EVRY suggestion in that Aricle, and EVERY SUGGETION in the Microsoft article it cites...and I HAVE updated TB with every new release....AND i have NEVER ONCE had a OAuth2 connection to IMAP server.

I will try your safe mode suggestion shortly. (I did run the tests described in my previous post after telling Norton Smart Firewall to remain off for 60 minutes.)

(FYI, In early June, TB showed me a message from Google server telling me I needed an App password. Which I obtained and use with no problem. I just now tried resetting in TB my Gmail IMAP server setting to OAUTH2 as a test, but, after restarting TB, I am not receiving new mail from Gmail (though no error messages shown). So I guess neither of my email accounts has ever had a Successful IMAP connection with TB using OAUTH2, although, as I said, SMTP with hotmail via OAUTH2 works fine). . Thanks again for your help.

TB showed me a message from Google server telling me I needed an App password. Which I obtained and use with no problem.

With OAuth authentication you must use the main Google account password.

Thank you. I plan to run the safe mode test later tonight after returning from a doctor appt.

It looks like I should also try to re-run the attempt to connect with Gmail Imap using the original password. One confusion I have in trying this is: I don't know hot to tell TB to return to my original Gmail password. Looking in the password Settings I see many google and gmail saved entries over the years...(I think including the TB calendar add-on, etc.) and I don't know what they all mean,so: can I safely delete all the Google / Gmail entries without causing trouble? Or can you advise me how to just delete the TB app password currently in use? Here is a list I made (with hidden user name) of the current entries I see using TB setgtings:

https://XXUserHIDEXX@gmail.com (Google Calendar OAuth Token) created 2017

imap://imap.gmail.com (imap://imap.gmail.com) -- created 2024

XXUserHIDEXX@gmail.com (Google Calendar OAuth Token) created 2016

oauth://accounts.google.com (https://mail.google.com/) created 2019

oauth://accounts.google.com (https://www.googleapis.com/auth/calendar) (created 5/9/2024)

XXUserHIDEXX@gmail.com (Google Calendar OAuth Token) created 8/4/2016

oauth://accounts.google.com (https://mail.google.com/) created 6/3/2019

oauth://accounts.google.com (https://www.googleapis.com/auth/calendar) 5/9/24

oauth:XXUserHIDEXX@gmail.com (Google Calendar OAuth Token) 1/20/2X

smtp://smtp.gmail.com (smtp://smtp.gmail.com) 6/28/201X

I think including the TB calendar add-on

There is no calendar add-on anymore, the calendar is built-in since quite a while. Just keep the latest OAuth token for the calendar, and delete the other older ones.

Delete all the OAuth tokens for your Gmail email. Also delete the remembered passwords for Gmail imap, and smtp.

Thunderbird will then prompt you for a password when it's needed.

You have been brilliant in pointing out how to solve most of the problems. When you told me that 0AUTH2 needs the original passord, not the App password, a lightbulb went off. With your help, after removing old Google passords, setting the google acct to OAUTH2 worked great for IMAP and SMTP. Same approach to Hotmail...And for the first time ever myTB has been receiving and sending via hotmail using OAUTH2 and the outlook.office365.com url for IMAP. So I'm not sure why I couldn't get this to work for months before I switched to the app password with Hotmail...but the point is, TB and the servers are once again communicating. So thank you very much for that. Two problems have arisenm in part I think as a result of a couple program crashes by TB while I was working with settings, or something similar. Can you help me fix the following: 1) An extra view of Local Folders without the most needed subfolders entered the folder tree view, but the problem is that the new Local Folder arrival with missing many subfolders took over the position that is most useful to me. It inserted itself in the folder tree at the bottom of all the folders in my 2nd email account at the bottom of the Unified Folder display.

Unfortunately, the good, Original and useful Local Folders group can only be seen much further down the folder tree at the end of the All Folders display. (I need and rely on the Unified Inbox, Sent, Junk and Trash folders...I  could uncheck the All Folders option if "Sent Copies" was visible where it used to be under Local Folders, instead of only appearing in the Local Folders view visible at the end of the ALL Folders list that  shows below the Unified Folder tree.  

So that is remaining problem #1: How to restore sent copies to the Local Folder view that is visible at the bottom of the Unified Folder display (which now only includes "Outbox" as a subfolder of that Local Folders)?

Second problem is: I'm seeing a big handful of duplicate emails showing up in the Unified Folder Sent folder...I've cleared up most of this problem by deleting Smart Folders from the Mail Folder in the profile...but this repair fixes most, but still permits a handful of duplicates to remain, especially in the Sent folder. It's possible that the problem is aggravated by too many messsages in the Unifed Sent Folder (TB says 4,354), and I am planning to archieve and trim this considerably. But a month ago there were no dupes and the sort was working perfectly, so something else may be involved as well.

If you can help me solve these remaining problems, both of which arose in the past 2 to 4 weeks of intensive effort to solve the OAUTH2 problem...I could finally feel life was returning to normal after an extended nightmare. Again, thank you for your precise, knowledgable, careful, correct, and rapidly offered, help.