If the update stopped all incoming and outgoing then this is because the server you are using is not using up to date protocols. So it was necessary to lower the minimum setting back to that used in version 68*.

I presume the 'security.tls.version.min' is now set to 1 which is the setting used in version 68*.

You would then use the same pop server settings as you used in version 68*, nothing would need to change.

After changing the setting you would need to restart Thunderbird to force it to use the setting.

However, you do not mention anything about the server settings you are using nor the provider so I can't advise on that until you offer the information.

re :Yesterday I called my provider, who told me they don't use TLS at all, but handle security in-house in some other way. That sounds a bit odd.

Thanks T-H (1) I am assuming (but don't really know) that the update has caused my problem. (2) Yes, tls is now set to "1". (3) I don't quite know what version I was using before, but I have restore the settings I found when I went into them. (4) I also re-started TBird. Still no go. (5) Settings: (Incoming) Pop Mail Server mail.blackfoot.net port 110 [username] connection security: none authentication: password transmitted insecurely (Outgoing) smtp.montana.com port 587 [username] Authentication method: password transmitted insecurely Connection security: none I should perhaps mention that montana.com is a domain long since bought up by Blackfoot and functionally an alias for blackfoot.net. Their techie and I tried various combinations of mail and smtp formulas, but none worked, so that doesn't seem to be the source of the problem. As I mentioned before, we also tried add ing a new IMAP account instead of the POP, but without success. (6) I agree that handling security in-house sounds odd. Odder still is that I could not seem to draw out any specifics about the method supposedly used. For the rest, my experience with issues that arise when two separate outfits have problems interconnecting after some upgrade is that both tell me it is the other guy's fault (or responsibility) until the matter is mysteriously corrected.

https://support.mozilla.org/en-US/kb/thunderbird-78-faq

Thanks, Wayne, but the link I cite in my original question is in fact the relevant art of the FAQ you have directed me to - and I have already consulted this, tried out the suggested solution there, and still come up empty - as I say in my post. There has to be something else afoot, it would seem. Still looking for a solution. HWA

If you are not getting any error messages from the server then maybe your computer Firewall is blocking Thunderbird and not allowing access to the internet. It is possible it thinks the update is a new program and has got confused. A couple of years ago this happened to me when I was using Norton Internet Security which also looked after the Firewall. Not had it happen since, but it could have happened to you.

Exit Thunderbird. Please access your Firewall, which might be part of your Anti-Virus product. Thunderbird must be listed as an allowed program. If it is already listed as allowed, then remove, ok it and then go back and set it to allow Thunderbird and ok it. If Thunderbird is not listed as an allowed program then it needs to be added and set to be allowed. Can you confirm the Anti-virus you use and that the Firewall is set up to allow Thunderbird.

T-H: I think this unlikely to be the cause, since within the same program I have set up two IMAP email accounts at universities, and they are working fine. But, to leave no stone unturned, I have looked, as best I can, my Firewall settings: (a) The one in my System preferences (under Security & Privacy) I found turned completely off - probably because I found it interfering with something or other at a time in the past I cannot now recall. I turned it on again, finding Firefox and updater checked as allowed, and so added Thunderbird. Starting up Thunderbird again, however, the same problem persisted. (b) I don't really know what anti-virus arrangements I have on the laptop. There is something called Bitdefender Antivirus for Mac on it, and sometimes when starting the machine up I get a message that it has updated, or the like. But whether it is on or off or actually does any defending I do not know. There is an invitation to login, but I have no idea how to pursue that route. I would in any case be very surprised if it is causing the problem I am having. As a comparison, I started up Thunderbird 68, which I still have in my applications file. It worked with the other two (IMAP) accounts, but still not the private one with Blackfoot. I begin to think there is something else going on besides the new update's tls security protocols. Perhaps I will need to look into a new email provider. But if you have any further ideas how to continue with this one, please let me know. HWA

Trying to think of other possibilities: Is there something in the webmail account which allows you to select pop and it needs to be switched on/enabled?

Could port be blocked? macOS start the Terminal app from Applications/Utilities.

type: telnet mail.blackfoot.net 110 also test: type: telnet smtp.montana.com 587

found helpful stuff here: https://serverguy.com/kb/manually-test-connectivity-imap-pop-smtp-services/

T-H:

(1) So far as I can see, one selects a POP account when setting one up, and is stuck with it thenceforth. If I want to change to to anything else, it requires entering an entirely new account. I find no on/off toggle option anywhere.

(2) the telnet times out and cannot connect with either server. I even tried various permutations of server name.

(3) But I find that it connects all right with the IMAP account I have with a university nearby.

If I am not mistaken, this test eliminates Thunderbird as the location of the problem, since was not a party to this approach. The blockage pretty much has to be at the server end. Does this seem right to you? HWA

(1) So far as I can see, one selects a POP account when setting one up, and is stuck with it thenceforth. If I want to change to to anything else, it requires entering an entirely new account. I find no on/off toggle option anywhere.

You are correct. You cannot simply swap around server settings. But you can copy emails into 'Local Folders' mail account, then remove the pop account via 'Account Settings' > select pop account name > click on 'Account Actions' > select 'Remove account'.

Are you currently located in the US? If no, then blackfoot would auto block your access via any email client.

re :(2) the telnet times out and cannot connect with either server. I even tried various permutations of server name. To check that telnet is working ok. Test one of the imap accounts that is working. eg: telnet imap.gmail.com 993 , but obviously you would not use imap.gmail.com (unless you have a gmail imap account) - you would enter the server name which you are using. If you get a connection then you know telnet is working correctly and the data from original blackfoot test is a good test result even if it is a fail result.

If telnet is definately working for other checks, then either the ports are being blocked by ISP or your firewall/router.

Still thinking out a loud here :) You mention Bitdefender and I also wondered if by any chance you are using a VPN Are you / could you be using a VPN ? Blackfoot may not allow access via VPN. If you are using a VPN then stop using it and restart Thunderbird. https://www.bitdefender.com/consumer/support/answer/11311/

T-H: Thanks for continuing to think about this.

But I have been derelict in failing to report some progress (of sorts) on the problem. After about two weeks of trouble-shooting with techies at Blackfoot, one of them had the bright idea to ask me whether I was outside the the US. Before even answering him, I tried using a VPN (which I do not normally) that I have with the University of Montana and found that the Thunderbird started downloading messages as usual. When I reported this to the techie, he replied as follows:

"Over the last month we have had to broaden the restricted IP blocks to include most of South America, the Middle East, Africa, and a majority of Europe. Some entities have put together a very impressive “bot”, the brute force attacks on ISP email accounts and what we were seeing was like nothing we have seen before. Unfortunately these actions had to be put in place to protect all of our customers."

So that seems to explain the problem. Is this what you had in mind when you said they would auto block my access from any email client?

As a partial solution, they offered to allow special access to my IP - and it seems they may have done this, since I can now get incoming emails via Thunderbird without the benefit of the VPN. I am still trying to solve a problem with sending emails, and await word from them in reply to my question about this. Meanwhile, you remind me that I can try the telnet check with both servers and see what I get.

As I mentioned, I don't really know how Bitdefender got on my laptop or what it is supposed to do. (It seems not to be a VPN, but a virus check of some kind.) I tried to find out more by making an account with them, only to find that they wanted payment of some kind for a service I could not understand. I decided just to toss anything to do with the program I could find on my computer. Apparently I didn't expunge everything, though, because I now get frequent, annoying messages telling me that they cannot connect right to do whatever it is they do, and that I should contact them if this continues. But I don't want to contact them, I just want to get rid of the thing, so if you have any ideas how I can finish the job, I would be interested in knowing.

I can see that am going to have to decide just how to be getting my emails while abroad - which is about half the time. Webmail doesn't cut it, since I see no way to use it to send messages and to keep copies of my sent messages on my computer, the way I do with Thunderbird. And this is not even to mention the unwieldy interface. How, for instance, am I supposed to consult an older message in my inbox while I am sending another one from the "send" area? The screen also filled with too much "administrative debris." There are many such problems that explain my preference for Thunderbird, even though it is by no means perfect.

The experimentation I did early on with Blackfoot's techies included deleting the Blackfoot account  in Thunderbird and starting over with it. This somehow resulted in my having something like two parallel accounts now, one of which has all my stuff in "Local Folders" (which is where old Inbox is), and the other of which is downloading new messages into a new Inbox, but has no Sent folder (at least yet). I am presently seeing how I might merge all this into a single account again. I will try the move-and-delete method you outline, and perhaps it will work to this end in some way. 

HWA

The last part was strangely formatted for some reason. Here it is again:

The experimentation I did early on with Blackfoot's techies included deleting the Blackfoot account in Thunderbird and starting over with it. This somehow resulted in my having something like two parallel accounts now, one of which has all my stuff in "Local Folders" (which is where old Inbox is), and the other of which is downloading new messages into a new Inbox, but has no Sent folder (at least yet). I am presently seeing how I might merge all this into a single account again. I will try the move-and-delete method you outline, and perhaps it will work to this end in some way.