Do you have two-step verification on the account, which would require an app password for the account in TB?

No, two-step verification is not required. Do you think Microsoft Outlook has made it impossible for T-bird to communicate with Outlook? (Consolidating their evil empire.)

My Hotmail account works fine, but there could be interruptions in some regions. Do you have a VPN, or security/antivirus app that scans the profile folder or secure (SSL) connections? Try running Windows in safe mode with networking to bypass startup apps that could be interfering.

I have Malwarebytes, and various safety apps from Firefox. I don't know if it scans incoming email. I twice rebooted in safe mode following the online directions, and though I chose option 5 or F5 for networking, it could not see any networks in safe mode, so I could not find out if TB would work in safe mode.

Safe mode for a computer on wifi doesn't automatically allow networking, so you have to enable the network adapter manually. But I think you can accomplish the same effect by excluding the TB profile folder from MWB scanning:

https://support.malwarebytes.com/hc/en-us/articles/360038479234

This doesn't change the real-time background scan of MWB, or any other AV app.

The profile folder location is shown in Help/More Troubleshooting, about:profiles.

Before I figure out how to do all that (this being way above my pay grade!)... I don't understand why MWB would suddenly stop TB from receiving any email from Outlook, after literally years without such a problem.

This happens quite frequently: TB or the security app work fine together, until one or both of them update, and the result is an incompatibility.

It might also help to remove the passwords from Saved Passwords in Preferences, restart TB, and enter the account password when prompted.

My helpdesk is telling me that the organization (University System of NH) is responsible for my problem, but there might be a way to get around it. Here is what the tech person there says: Hi Judy, The change that was made earlier this week was to not allow what Microsoft refers to as “Basic Authentication”. My guess is that Thunderbird’s POP set up for Exchange runs off Basic Authentication, or at least how you currently have it configured. If you have Thunderbird support already contacted and are working through a thread with them, I would ask if they can send instructions for configuring Thunderbird using Modern Authentication. That is the key, ultimately. Basic Authentication is a security vulnerability that USNH ET&S is mitigating a couple months earlier than Microsoft plans to decommission it. I believe Microsoft is turning off Basic Authentication as an option in February, so we’re trying to get ahead of it as an organization. Hopefully that helps – Modern authentication and see if they have ideas. I did google “Thunderbird and Modern Authentication” and saw another school’s online knowledge base article…I didn’t look too closely at it, but it made me at least think it was possible…. Elise

Microsoft accounts with organizations work a bit differently from consumer hotmail or outlook.com addresses.

You can try changing the authentication method to OAuth2, remove the passwords from Saved Passwords, restart TB, enter the account password in the OAuth window when asked to allow access. Cookies must be accepted in TB Preferences for OAuth to complete.

https://office365.mcmaster.ca/reconfigure-mozilla-thunderbird-for-modern-authentication/

Try changing the authentication method to oAuth. If available.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1528136

Thank you, sfhowes and Matt. I changed authorization method to OAuth2 for both incoming and outgoing servers, and that did the trick.