What mail provider? What settings do you have for the outgoing server (SMTP) that you are using to send mail (not the incoming server details, the SMTP server for the identity).

It woulds like you have not completed the oauth authentication for your providers outgoing (incoming and outgoing are two different authentications) server and are complaining that it does not work, but I am trying to read between the lines with very little actual information about what the problem is.

have you lost anything, if you use IMAP to get mail, if the account is not connecting you will loose everything except an inbox and a trash folder until the connection is restored. It is not a real data loss, it is a red flag you have a connection issue. Perhaps Thunderbird should not delete what it can no longer validete with the server, but it does. Once the connection issue is fixed the folder will reappear in most cases.

You downgraded? Really. Was it to the ESR version which updates about once a year or to the release 140 default which will just start you on the update merry-go-round again? Did you loose you profile and contacts when you downgraded? it would be expected behavior.

While I have some personal opinions about the release versions, I can not really fault the ESR version I stayed with (140.5.0esr 64 bit). It suits my needs, has most addons working with it and only updates for critical, mostly security, bugs each month.

Thanks for the replay Matt.

My system is: Win11 Pro 24H2 - OS Build 24007.71

The the last several weeks I have been beating my head against a wall trying to figure out what happened when TB upgraded to 141.

141 - 8/13/2025: I had messages that would not delete.

142 to 143 upgrade 10/7/2025: Half my folders disappeared; I still could not delete any messages or folders; I could not create a folder. Then I realized that there are TWO versions of TB: ESR and RELEASE.

Current: Now, I'm on version 145.0, half my folders disappeared again. HOWEVER, I have figured out why I wasn't able to send anything. The outgoing SMTP.Comcast.net authentication method was set to OAuth2 instead of Password. When I changed it back to "password" I was able to send emails.

Weird Part: My 'missing folders' are listed in the root directory user/app data/roaming/xxxxxx/thunderbird/profile esr /mail / mail.comcast.net / inbox / xxxx

                  BUT they will not show when TB is open.

Thanks for the replay Matt.

My system is: Win11 Pro 24H2 - OS Build 24007.71

The the last several weeks I have been beating my head against a wall trying to figure out what happened when TB upgraded to 141.

141 - 8/13/2025: I had messages that would not delete.

142 to 143 upgrade 10/7/2025: Half my folders disappeared; I still could not delete any messages or folders; I could not create a folder. Then I realized that there are TWO versions of TB: ESR and RELEASE.

Current: Now, I'm on version 145.0, half my folders disappeared again. HOWEVER, I have figured out why I wasn't able to send anything. The outgoing SMTP.Comcast.net authentication method was set to OAuth2 instead of Password. When I changed it back to "password" I was able to send emails.

Weird Part: My 'missing folders' are listed in the root directory user/app data/roaming/xxxxxx/thunderbird/profile esr /mail / mail.comcast.net / inbox / xxxx

                  BUT they will not show when TB is open.

Why am I doing this and not TB when it upgrades?

Just found this link about OAuth2 vs password for outgoing email servers:

https://www.xfinity.com/support/articles/update-your-xfinity-email-settings

I install TB as POP3

One more thing Matt: I just received your email stating that you replied to my posting.