If the current passwords are removed from Tools/Options/Security/Passwords/Saved Passwords (Options/Privacy & Security/Passwords/Saved Passwords in TB 78), then TB is restarted, a password prompt appears at the next connection. Enter the account password and check the box to have it stored in Saved Passwords. If two-step verification is on the account, create and enter an app password instead.

Thanks for the help. Sorry to say that things have gone past that stage. First, the old imap and smtp settings were deleted in an attempt to fix things. When I enter what appears to be the correct settings I get the password request box, which when filled in with the correct..works on every other machine.. I get "Sending of password for user xxx@outlook.com did not succeed. Mail server outlook.office365.com responded: Logon failure: unknown user name or bad password." There appears to be no wizard that allows one to edit the system without it creating a new account, even though it says it is working on an existing account, it always generates a new one, leaving the old one with the same name in limbo, and complaining about there being two accounts with the same name. All I want to do is get in and edit the existing account details.

Thanks for the help. Things are past this now. The original account setting were changed in an attempt to fix the problem, because whatever I do, even if I get the password request up, it returns "Sending of password for user xxx@xxx.com did not succeed. Mail server outlook.office365.com responded: Logon failure: unknown user name or bad password." Password known to be good on every other machine. When I use the Edit existing account wizard, it always generates a new account, there appears to be no way of editing an existing account to replace the existing profile details or whatever. It should be possible to delete the existing password and force a new one, keeping all the existing messages otherwise.

I have a very similar problem. I setup my outlook 365 account in Thunderbird using Owl for Exchange two years ago and it worked quite well daily. I shut down my Thunderbird one or more times daily, starting it at the beginning of the next day with no problem. Suddenly on Wednesday 13 January 2021 the account I had setup for outlook 365 disappeared from the folder pane.

O/S Win7 Home (All updated) TBird V78.6.1 Owl for Exchange plug-in V 0.7.7.1

I am also having the problem with automatic filter operation. The filters work very well when selected manually after the messages are placed in the inbox. (See "Message filters work manually, but not automatically. This is new." - user jbeech)

@OldWilly Please stop hijacking other folks issues to present your own. If you have an issue and the information in the topic does not fix your problem, ask your own question. You jumping in only confuses the issues for everyone.

I have a very similar problem. I setup my outlook 365 account in Thunderbird using Owl for Exchange two years ago and it worked quite well daily. I shut down my Thunderbird one or more times daily, starting it at the beginning of the next day with no problem. Suddenly on Wednesday 13 January 2021 the account I had setup for outlook 365 disappeared from the folder pane. O/S Win7 Home (All updated) TBird V78.6.1 Owl for Exchange plug-in V 0.7.7.1 I am also having the problem with automatic filter operation. The filters work very well when selected manually after the messages are placed in the inbox. (See "Message filters work manually, but not automatically. This is new." - user jbeech)

Sorry, Matt. It was not my intent to highjack this thread. I did have pretty much the very same problem as the originator and in the same general time frame with regard to the outlook 365 account. I would not be surprised if he had also applied the Windows update that was causing my problem. I felt it would be helpful to also mention at the same time I had the problem reported by jbeech about the filters not working correctly, also the same time frame, thinking perhaps there was a common DLL or the likes that were related to the same problems.

I am done with this problem.

As an update, there was no posted solution to the problem of TB Outlook link to Outlook mail stopping working. To summarise, despite removing the previous password and multiply checking the validity of the new POP3 password, I kept getting a failed login warning from TB. This was the day after a TB upgrade. I rolled back TB by one edition, and for some reason, 24 hours after setting the password the link started working "all on its own". So the only advice I can give for others is clean out the old password, reset the password with a new verified version, close the computer, wait 24 hours, and hope. Somebody obviously fiddled somewhere, but nobody is going to admit it. When I have the energy I will allow TB to update, and hope that it does not immediately fall over. If it does I will find another mail client.

My understanding it there has been yet another release of OWL 0.7.7.5 was released on the 20th January. I guess you have that update not and the problem "went away"

It was NOT the Owl for Exchange that caused the problem. As stated earlier, it was the "Windows Malicious Software Removal Tool (KB890830)" that permanently corrupted the profile. I solved the problem by doing a system restore from a restore point just prior to the Windows update.

I am done with this problem.

As noted, I rolled the TB version back from the version which installed itself immediately before the problem, and I am running 78.6.0. until my nerves can face the prospect of dealing with 5000 hung emails again. Thanks for the attempt to help.