This could be a problem with the server's configuration, or it could be someone trying to impersonate the server.
   If you have connected to this server successfully in the past, the error may be temporary, and you can try again later.

Why is the following displayed in the message display area of Thunderbird whenever I start Thunderbird? Secure connection failed www.mozilla.org uses an invalid security certificate. The certificate is not trusted because no issuer chain was provided. (Error code: sec_error_unknown_issuer) This could be a problem with the server's configuration, or it could be someone trying to impersonate the server. If you have connected to this server successfully in the past, the error may be temporary, and you can try again later. Or you can add an exception… Selecting the link to "add an exception" is dead, and doesn't display anything additional.