Is it IMAP or POP?

If IMAP then there is no real need to rescue everything because it is on the server. And you could log in to the webmail portal offered by your email provider to check what's there.

You say "GTE/Verizon/AOL email account" - is this one account or several? How come they are all broken?

When you delete an account, if that's how you decide to go, at the point of deleting it you are offered a choice about deleting just the settings or both the settings and the data. If you choose to preserve the data, then you can set up the account again and then re-select the "Local Directory" that the original account used.

I rather think that advice to delete and re-instate accounts is something of a desperate guess at what to do. It would be my last resort, though I have to say that I have done this myself recently with an IMAP-connected outlook account which had probably failed because the smtp server had been reconfigured, and there is no easy way to get Thunderbird to discover the settings without tearing down and setting up the account afresh.

Whilst no messages are lost, it's a pain to have to repeat all the little tweaks one does; filters, folder layout, address book association and de-association. Some of this might be restored by reinstating the original Local Directory.

Thanks for the reply. It is one email, the only one I've ever had, now close to 35 years -- mfa@gte.net. GTE became Verizon, and Verizon sold their email business to AOL, thus the chain. A couple days ago AOL announced they had made changes requiring me to upgrade my settings in Thunderbird. After making the changes, I could no longer send email. In fact, I'm not sure if the old settings still worked either, but in any case AOL support gave up after an hour and a half of effort at two levels of support. Ultimately, they suggested I hire a 3rd-party support service, implying strongly that Thunderbird was the problem, and since I could send email using AOL's web email service, they were washing their hands of me. I have to admire their chutzpah, since they sent me a "how was our service?" survey today.

I use POP, mostly since I do a LOT of searching of archived mail, and it's faster to do it locally. I agree it's likely that their suggestion was a last-ditch effort on AOL's part. With your helpful advice, I'm planning now on saving my entire profile and deleting the account settings, and see what happens. At this point I'm not sure what will be saved and what will be deleted if I delete "settings only", but I'll see.

What would happen if I reinstalled TB altogether, and, using the default profile, set up the email account again. Then I could test it very quickly, and if it worked, somehow graft it to the data in the saved profile? I can rebuild profiles, etc., without too much effort, but the archived mail folders are critical.

If it didn't work, then I could at least go back to AOL, toss the ball into their court again, and see if they can make it work with a fresh copy of Thunderbird.

You can tell I'm new at this.

I would suggest:

1. Create a new blank profile.
2. In this new profile, set up your account. Hopefully Thunderbird will discover the appropriate settings.
3. Note them down, for both POP and SMTP.
4. Restart Thunderbird using your old profile.
5. Change your settings to the new ones.

The main snag is that if there is a lot of material waiting to be downloaded, it may all end up in the new profile and not easily accessible to you after setting Thunderbird to use the old repaired profile.

https://support.mozilla.org/en-US/kb/using-multiple-profiles

Thanks, that's more-or-less what I had planned to do, although I was going to start with a new install rather than a new profile. I've got to do a little more investigating to see what the difference is, and what's in the profile (or not), but will start with just a new profile. There should be no mail waiting to be downloaded -- it's only sending mail that's been failing.

New profile created the account automagically, and everything worked fine. I then started TB using the old profile, and copied the new POP and SMTP server settings to the old account. Again, I could receive emails OK, but old sending problem remains. Message eventually displayed (unchanged) is:

"Sending of the message failed. The message could not be sent because the connection to Outgoing server (SMTP) smtp.aol.com was lost in the middle of the transaction. Try again."

I followed digital breadcrumbs to MozillaZine's "Connection errors - SMTP" (last modified 9/2011) but their only mention of this error message was:

"Its a bad idea for your anti-virus software to scan outgoing messages, especially over a secure connection. That or a problem with some type of security suite can cause an error message such as: The message could not be sent because the connection to SMTP server server name was lost in the middle of the transaction. Try again or contact your network administrator."

I'm using WIndows Defender and MalwareBytes. Neither of these explicitly mentions scanning outgoing emails, but I vaguely remember that feature being mentioned somewhere for some reason in the past. I'm inclined to ignore that issue, though, since things DO work OK using the new profile. Rebuilding the account definitely fixed the problem, but as I understand things, I can't rebuild the account in the old profile.

Looks like I've got to copy the new account code into the old profile or the old data folders into the new profile. I feel better about the latter; not sure how to go about it, but we'll see.

I think I'm good. I created a new profile, added only the mail accounts I'm using, and let TB auto-configure them. That worked to solve the original problem.

Then it was just a long, arduous process of figuring out where to copy various mail files from the old profile to the new one. .spd? .spf? smartmail? duh. But basic email is now working, probably better than it did in the past, and I can rebuild the filters and folders I'm used to.

Still some minor glitches, but I'm encouraged. Thanks for your help.