I am getting the following screen when I open Thunderbird. Add Security Exception You are about to override how Thunderbird identifies this site. Legitimate banks, stores and other public sites will not ask you to do this. Server Location: [pop.gmail.com995] (a box) [Get Certificate] Certificate status This site attempts to identify itself with invalid information. [View] Unknown identity Certificate is not trusted, because it has not been verified by a recognized authority using a secure signature. [✓] Permanently store this exception [Confirm security exception] [Cancel]

What is this and do I have to be worried about it? How can I stop it permanently?