Try setting it to one?

Matt, Actually, I did try 1 for Gmail and it still happened, so I took it back up to 2. Couldn't remember if that was before I deleted TWC (which was also 'throwing' an Inbox unknown command error). I just tried it again, all three set to 1 and still get the error.

Kurt

Is it possible you have more than one device connected to these accounts? Tablet, phone, laptop... If you are still logged in through a different device, log out and wait a while, then try Thunderbird again. If those devices auto-login to email, you may need to take them offline, or turn them off for a while before trying Thunderbird again.

The error on TWC is harmless and a result of spectrum having a badly configured mail server. It announces is supports something and then tosses out an error when Thunderbird uses it. It does not bode well as it has been fundamentally broken for years now.

Use the Config Editor to set mail.server.default.send_client_info to false (right click the preference in the Config Editor to get the menu of actions. See Bug 741374 for the source of the pref. I have no idea where it started.

Does the error identify a specific account or accounts? (a screen capture of the error message would make it crystal what it does not not show.)

Agnes, Only my phone checks Gmail - when I occasionally connect to WiFi and Data is always off.

Matt, All of this only started this year. I thought Spectrum was the culrip with the two errors, but after deleting, the Imap error continues. Love the warranty warning for Config Editor! No, the error doesn't specify which email account... guess the next step is to delete two account and see if the error goes away. I'm trying to capture the error, but it pops up on the notification area and goes away as soon as I click outside of Thunderbird. Just saw you sent another message... send this and trying to catch error.

Matt, OOPS, saw both messages and only opened Agnes'.

I was able to catch the error: attached.

Thanks for the suggestions! I'll get back to you all later... need to swap out a monitor and ALL my desktop stuff collapsed onto one monitor, so it's going to take a while to fix. I will try to delete each of the accounts and see if I can narrow it down. I'll reply when I have more info to share.

KT said

I will try to delete each of the accounts and see if I can narrow it down. I'll reply when I have more info to share.

Before you do that, have a look at the error log (ctrl+Shift+J) it is full of all sorts of rubbish, so will take a little wading through, but it also reported just about everything.

The error does look like it might simply be a guess on Thunderbird's part as to what is wrong. At this point concentrate on unable to connect to IMAP server.

This could be as simple as the anti virus choking on the flurry of get mail activity going on.

Are you getting multiple master password prompts by any chance?

Matt, There were very few items that had a time stamp... and nothing related to the imap error that I saw. Usually when I open Thunderbird, there may be a few emails (0-5) but not a lot. There were a LOT of 'errors' for Gmail Calendar related to different 'style' issues that were ignored... perhaps that is what is causing the connection problems... I'll try removing the Calendar and see what happens.

Chrust1, No, not getting any password prompts.

All, After upgrading my Linux box to an SSD and reinstalling Mint 19.3 Xfce, I setup Thunderbird on it to read-only Gmail without any issue, so I may use it as a testbed, adding the other accounts to see if I can get it to give me the error... apples and oranges comparison perhaps.

Kurt

All, Seems it was Google Calendar causing the IMAP errors.

I created all of the email accounts in Thunderbird on my Linux box without any problems (5 IMAP connections each).

Went back to my Win10 box and closed the Calendar tab and restarted... no more error.

Added Calendar back, then unchecked 3 of the six calendars and no error. Took each of the server connection back up to 5 each, no error.

I had a couple of calendars checked that I don't even use, as well as Contacts was listed twice. I've unsubsribed from the unused calendars and the 'extra' Contacts.

Thanks for the help and suggestions!

Kurt

FYI to all, After the Windows 10 update to 2004 (June 2020), the error has finally gone away, so it would appears it was a Microsoft issue, perhaps with the way the OS was parsing the IMAP request????

Thank to all for the suggestions! Kurt