When I try to subscribe to the Samba mailing list at https://lists.samba.org/mailman/ I get the following message in Firefox: "Secure Connection Failed lists.samba.org uses an invalid security certificate. The certificate is not trusted because the issuer certificate is unknown. (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." Is this normal?