uncheck the option for 'allowing immediate notification....'

I unchecked 'allowing immediate notification....' - no change, Tbird continues to check for new email. ALL of my email accounts have ALL the Server Settings UNchecked, including "Check for new messages at startup" and "Check for new messages every..." Is there another setting I'm missing? Any other suggestions?

make sure Thunderbird is not in one of the IMAP folders when it shuts down. On opening it will reach out to the IMAP server to refresh the mail that is listed in the folder, the same as if you clicked on it.

I also suggest that you set Thunderbird to offline mode before you close it down. Nothing stops outreach like being told the internet is not connected.

Matt: thanks, I tried that and soon as I restart Thunderbird, it's back to checking emails. I put Tbird in offline mode, closed and it comes back on "by itself." This is becoming frustrating but there doesn't seem to be a way to stop it. It seems to be a minor irritant but, switching between laptop and desktop as I travel, this is a pain.

There has to be some other setting I can't find...other ideas?

If you always want to start Thunderbird in Offline try this: Settings > General scroll to 'Network & Disk Space' section click on 'Offline' button

uncheck 'Automatically follow detected online state' Manual state when starting up: select 'Offline' click on 'OK'

re :ONE of the IMAP accounts started checking email on startup

Maybe that particular server uses PUSH/IDLE eg:Gmail uses PUSH if IDLE is supported and it means gmail will open connection and force the download to allow imap account to reflect up to date situation on server.

This is an explanation of situation: If the three items are disable (not ticked/checked) the account is almost disabled. However, if you select (visit) a folder (e.g., INBOX) any new mail will still be fetched. So to effectively disable an account you have to not select/visit any folder of the account that can receive new mail.

Sounds crazy, I know, but that is the explanation I received. After all, just because you do not want to get messages does not mean you do not want to read messages previously received.

However, there is an addon which may help:

I found this info: https://bugzilla.mozilla.org/show_bug.cgi?id=334483#c45

Filtaquilla - it offers a do not notify option https://addons.thunderbird.net/en-us/thunderbird/addon/filtaquilla/