What exactly are you trying to achieve?

I do not want Thunderbird automatically synchronizing with my online AT&T account. As with POP3 accounts, I want to be able to download messages from the online account to my local folders, but only at my command. There should be a simple way to turn off the automatic synchronization.

Open Account Settings and change server settings accordingly. You'll see the settings that control automatic fetching of messages at startup and periodically.

I do not want Thunderbird automatically synchronizing with my online AT&T account.

If you mean you don't want Thunderbird to be a just like a mirror of what's on the server, then you shouldn't have used IMAP in the first place. It's just how IMAP works. Set up your account as POP instead.

The Thunderbird definition for 'synchronization' is downloading an offline copy of your messages to your local disk. You can disable that in your account settings. https://support.mozilla.org/kb/imap-synchronization

Stans, I have tried to turn off synchronization in the Server Settings. It does not stop synchronization at all. I also tried to turn off Message Synchronizing under Synchronization & Storage in the Settings. That had no effect either.

christ1, it appears you are correct, but I have a problem with your answer. I set up this account in Thunderbird several years ago as an IMAP account as per AT&T's instructions. I did not have this problem (forced synchronization) for a long time until about 6 months ago. I don't know if this activity was triggered by a Mozilla update or some change in AT&T's settings. I have since updated TB to the newest version, but the situation continues. Of course, I suspected IMAP was the problem as I have several POP mail accounts set up in TB, and they don't act the same way. I just thought there must be some way to stop the mirroring.

After doing some research, I discovered that AT&T does have a POP option. I've set up a new account with that server setting and if that does solve the problem, I will delete the IMAP account in TB. Hopefully, that won't trigger AT&T to delete my account with them.

I set up this account in Thunderbird several years ago as an IMAP account as per AT&T's instructions. I did not have this problem (forced synchronization) for a long time until about 6 months ago.

What exactly happens when you say "forced synchronization"? I have a feeling you're describing something else entirely due to lack of better terminology. Nothing has changed with the way IMAP works.

Forced synchronization means that I apparently cannot turn it off. If that is the way IMAP is supposed to work, then fine. But it had not been doing that (mirroring) until a few months ago, and now it is. I did not make any changes in the settings then, and now I can't disable synchronization. In my book, that is the definition of "forced".

Perhaps nothing has changed in the way IMAP works, but the way Thunderbird controls it certainly has.

I can not begin to comprehend what you had or though you had going backwards. But the whole basis of imap has been since the protocol was first ratified in the last century to synchronize mail locally with a server store. The protocol is so "synchronize" centered that I do not recommend folks even consider mail in IMAP account on their local machine as local copies. They become local copies once the are moved to Local folders because they are no longer in the IMAP account, but until such time they are really only a local cache to speed up display of remote mail. I am guessing that you may also have the "allow immediate notification when new messages arrive" in server settings for the account enabled. This will get mail once a connection has been created.

If you want the ultimate control over when mail is fetched (something that is somewhat pointless unless your on a very very slow internet connection) manage the online and off line status by clicking the icon in the very bottom left of the Thunderbird screen. When it has a red mark on it it is offline and no internet functions will work and you will be prompted to go online to send mail for instance like we used to 30 years ago on dailup modems.

I can not begin to comprehend what you had or though you had going backwards.

Me neither.

But it had not been doing that (mirroring) until a few months ago, and now it is. I did not make any changes in the settings then, and now I can't disable synchronization...Perhaps nothing has changed in the way IMAP works, but the way Thunderbird controls it certainly has.

You seem pretty certain about that change in Thunderbird, but at the same time you say you didn't change any settings then or now and so you can't say for a fact that it's some kind of missing control/setting that got removed in your current version, and that Thunderbird behaved that way out of the box without your interverntion. I'm more than happy to verify these claims for a fact and for the record; all you have to do is state which version of Thunderbird you were using that behaved differently. All versions of Thunderbird are archived here https://archive.mozilla.org/pub/thunderbird and anyone, can confirm or debunk those claims. It is of no benefit to anyone, for us to refute a certain change in Thunderbird. If anything, changes are documented in release notes for every version of Thunderbird.

Thanks guys for all the help. Still don't know why there appear to be ways to turn off synchronization that do not work, especially if IMAP forces synchronization. Also still not sure why synchronization wasn't working up til a few months ago - must have had a broken email client - or as you pointed out, I must be crazy.

But at least now I know better.

I just switched to the POP server for AT&T. Problem solved.