Hello there! It sounds like you're encountering an issue with Thunderbird. Don't worry, we've got you covered. This error usually occurs when you've reached the maximum number of connections to the IMAP server. To fix this, you can try adjusting the settings. Go to Thunderbird's preferences, then to the account settings for the affected account. Look for the "Server Settings" section and click on "Advanced IMAP Server Settings." There, you can tweak the number of cached connections. Reduce it a bit and see if that helps. If the issue persists, you might also want to check if there are any add-ons causing conflicts.