You're seeing a warning message from your email provider, not from Thunderbird. More information is needed to make an educated guess... - Who is your email provider? - Is this a POP3 or IMAP account? My first thought is that you use a POP3 account. If set up a certain way, Thunderbird would only download to your local computer what it considers to be new messages, leaving copies of them on the server. If so, they could have been accumulating there out of visibility to you for some length of time. The solution is for your go log onto your web mail page and see what can be deleted from there.