Then you have not closed all the cached connections. Exit Thunderbird. This will force all the cached connections to close. Do you have any other device accessing the mail account eg: another laptop, desktop, phone etc ? If yes: If another computer has Thunderbird or Outlook etc then you need to exit those email clients on those computers. If you are using a phone to access emails then you need to switch it off. Phones are the most common likely cause because they have a habit of keeping connections open. Suggest you wait for a short while before restarting Thunderbird, this is because it can take the server a bit longer to recognise all the connections are closed.