What version of Thunderbird do you have? It would normally upgrade itself automatically unless you have turned off all updates. If you did, and the version you have is quite old, then upgrading may take several steps. Go to Help - About and you will see the current version, and it will say if it is updating. Version 91 is the current version unless you are a beta tester.

If you have an intermittent internet connection failure you could see that warning, but then as the connection resets the email continues to work. Do you know of any issues that might interrupt or slow down your Internet service?

How do I get to the recommended "Help-About" location?

Search in this support forum for how to find out your Thunderbird version, or how to update Thunderbird. You'll find fully illustrated instructions. In brief, Application menu button (looks like a hamburger,three horizontal lines), then down to the Help menu and choose Troubleshooting or About. If your version is really old it may not look like a hamburger - look for the word Help in the top menus.

First, I appreciate your trying to help. However, I am not a computer software engineer. I have looked but I have never found an "Application menu button". I do not believe that my software is extremely old because I recall an update being done about a year ago. Doing a general seach produces nothing of value. What page/site is the "Application menu button" located?

It is in the top bar of Thunderbird. Right hand side. If you search this support forum for the articles I mentioned you would see pictures to help you locate it, which is why I suggested that. Try typing 'find out your Thunderbird version' in the search box on the same page as this reply...

My "hamburger" is three straight horizontal parallel lines. I went to "help" and "more troubleshooting" and it says "version 91.1.2" Is this my version or just the latest version available? When I search "find out your Thunderbird version" it simply returns me to the messages that you have sent. Very frustrating.

Ok, yes that is your version. So now you can tell Earthlink that you have updated and are on the latest version. You do get incoming new email which proves Thunderbird is connecting to their server some of the time, so either their end or your end loses the connection occasionally. The error specifically says their server disconnected. If you also have trouble with slow loading of other Internet pages, the problem is probably at your end, or with the provider of your internet connection. If email from Earthlink is the only service affected, it could be their server is overloaded or losing connection. I don't have any more ideas for you - but you are one step further now in knowing Thunderbird is up to date and working.

Thanks for the guidance and help. I will contact Earthlink again and see if they can diagnose the problem. Thanks again!!