@mikes2400,
Just some other questions for you,
If it's jut thunderbird you are having issues with, would you be able to provide any information at all such as dates and times?
Does it also happen with all emails or is it a specific email at all?
Have you removed your email account from the thunderbird and then re-added it to see whether you still get the same error message?
Are you also able to replicate it on other devices such as 3rd party apps? Just so we can see if it is just thunderbird you are having this issue with?
Thanks