Where is your Thunderbird profile folder located? Given you have issue and million don't I am thinking you probably have a non standard location for either your mail or your profile. Then you say you used Eudora and old Eudora users almost always want to store things somewhere other than the default. Seems to go with the territory.

Goto the troubleshooting information on the help menu and click the open folder button. That will open the folder in windows file explorer so you can see where it is.

Thank you for your help! This is what I found: C:\Users\Victoria\AppData\Roaming\Thunderbird\Profiles\hid4pw25.default

??

--Victoria

That sounds about normal. SO much for that bright idea.

Now have a look in your account settings for each account.

At the bottom of server settings (you might have to scroll to see it. I do) is a local directory. Do these all start with C:\Users\Victoria\AppData\Roaming\Thunderbird\Profiles\hid4pw25.default

I would expect things like /mail/server names and \imapmail\servernames really but non standard I was thinking one or all might point o documents to a cloud backed up folder.

Thank you. They all (3) start with the same profile.

Is Thunderbird the only program affected by this behaviour on your system?

Thunderbird is the only program that won't open properly. Other programs, such as Firefox, start being very sluggish because Windows Defender SmartScreen is hogging resources. But TB is the only one that can't open, etc.

I don't know if this is related, but Google will not allow me to directly interact with a newer (last 2? years) email address that I created. I cannot collect email for that address, and Google will not forward the email for that address to me-- it sends me error messages that the username and password are invalid, but the thing is they are identical to the ones I can log directly into gmail.com with. The newer address is set up with identical Server Settings, Outgoing Server, everything is identical, but Google won't let me use it in TB. I prefer Pop mail because I want all my email in one place, where I can manage it directly, not floating copies everywhere. My working life is set up that way and that's what works for me. But I wonder if gmail may be refusing TB because of the pop mail settings. In any case, this email not working is not an issue, the email is only occasionally needed. But I wonder if the email issue is somehow related to the SmartScreen/TB conflict.

I don't see any connection between the new Gmail account and SmartScreen. We can tackle that once Thunderbird or your computer is running as it should. This may not be a Thunderbird issue at all, but a Windows problem, specifically your Windows setup.

SmartScreen should not be freaking out on Tbird because Tbird is not a low-reputation program, and has a trustworthy digital signature UNLESS your copy of Tbird has been compromised, or the files it's accessing are infected, in which case you should be getting an alert to that effect. Let's see if a new profile makes any difference in startup performance. Quit Tbird then open its Profile Manager as described here Profile Manager - Create and remove Thunderbird profiles

Create a new profile, but don't make it the default by unchecking the option that says "use the selected profile without asking at startup" then start Tbird using the new profile. Since it's a new blank profile with no accounts, add-ons or other data to load, Tbird should start right up. If it does without much of a fuss from SmartScreen, then we can conclude that your old profile is the culprit. If it doesn't start right up and exhibits the same behaviour, we can rule out the old profile as the cause.

It worked!! Thank you!! SmartScreen did engage, per inspection at Task Manager, but Tbird opened right up. I went ahead and put in my main email and password. So now all my old emails/folder structure, settings, add-ons, etc. are not visible in this new iteration. Can you help me reclaim all my old material etc? Thank you so much! I'm making a donation to Tbird today.

wow

So now all my old emails/folder structure, settings, add-ons, etc. are not visible in this new iteration. Can you help me reclaim all my old material etc?

Those are in the old profile. Let's see if adding the old profile's folder C:\Users\Victoria\AppData\Roaming\Thunderbird\Profiles\hid4pw25.default to Windows Defender's exclusions will allow Tbird to load it without interference from SmartScreen. This is recommended, because interference from security programs is a well known cause of trouble for Tbird.

For steps to follow, go to https://docs.microsoft.com/en-us/microsoft-365/security/defender-endpoint/microsoft-defender-security-center-antivirus?view=o365-worldwide, then start the Profile Manager and launch the old profile.

Thank you! This worked, and the problem appears to be solved. I really appreciate the help getting it set up so I didn't have to figure out all my settings and add-ons again. Phew! Once again the great people at Tbird community support have come through for me. Thank you so much, Stans. All best wishes. --Victoria

You can delete the new test profile now and make the old one the default so that you don't have to choose which one Thunderbird should load at every startup.

I didn't actually delete it but I did return to the default and made it load automatically. My computer is still slow, but at least I'm set up to optimize Windows Defender compatibility with Tbird. THANK YOU!!!!!