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.

I got this message and do not know what to do I can also hit exception to receive an email but not sure if this is safe, Help! Secure connection failed start.thunderbird.net uses an invalid security certificate. The certificate is not trusted because the issuer certificate has expired. (Error code: sec_error_expired_issuer_certificate) 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.