Trying to access non existing account

Some things to check...

- Check your calendars and address books to see if they are trying to synchronize with unknown, unremembered, or suspicious providers; Also see if you have any add-ons for syncing calendars, address books, etc.

- Check the setting <Settings | Composition | Attachments> and see if it's set to offer to share files at a storage provider.

- Maybe you have an open email that's trying to access icloud? Check <Settings | Privacy & Security | Mail Content>. Is "Allow remote content in messages" enabled? That's a general security risk, although there is the "Exceptions" button for you to indicate trusted sources.

Appreciate you replay but see below.

I have no synchronization of any kind, never have. Don't use calendars of any kind. Never used any kind of storage other than the two longtime email accounts which hold emails on their servers. I don't use POP or any Microsoft systems. Have no add-ons. Allowing remote content is done individually when reviewing emails. Is not automatically allowed.

I have never had anything to do with iCloud period. It's just one of my accounts that keeps saying can't access iCloud, needs it's iCloud password. Since never setup an iCloud email account and have never added a Thunderbird account for iCloud am at loss. Can't access iCloud, Apples ID system is a mess, gave up long ago on signing on to it.

It seems to have started upon having to reboot the computer, hence all programs. Seems to happen at Thunderbird start up with automatic checking email accounts.

If you have checked to make sure your server settings are correct, according to your email provider's recommendations, I'm wondering if something is running in the background that has inserted itself between Thunderbird and the network ports it's trying to use. This could be an anti-virus program or malware. If you use an AV, can you temporarily disable it for testing purposes? Do you use any other "helper" programs, like a separate spam filter, or a VPN? Or does your home gateway/router have any port forwarding instructions defined (something you would have done yourself, so probably not.)

You can try restarting Thunderbird in Troubleshooting mode, which temporarily disable all add-ons and may indicate whether that's the source of the problem. See Help | Troubleshoot Mode.

You don't mention your computer platform, but in Windows you can use Task Manager to get an idea of what is running when you get the error message, which might give a clue. There's also a more techie utility by Nirsoft called CurrPorts that will tell you what local programs are accessing what ports. Other operating systems probably have equivalent utilities.

That's all I have for now. Very mysterious.

What I have concluded is that this is coming from the OSX itself, not Thunderbird. Reason is the same issue was first occurring on my AirBook. I removed Thunderbird since I didn't really use the laptop that often.

But upon boot up it occurred again on the laptop.  And stopped, so far, on my Mac Mini which is where I do my email.   In essence the error message says "Computer can't contact iCloud due to not having my "primary email account" 's password.  If no Thunderbird installed then has to be coming from the OS itself.  Just another example of Apple trying to stick it's nose into someone's business.