My Thunderbird (v31) appears to leak my IP address ... or is it my Postfix server? anyway, it sounds bad regarding anonymity. Is it a configuration problem? my

My Thunderbird (v31) appears to leak my IP address ... or is it my Postfix server? anyway, it sounds bad regarding anonymity. Is it a configuration problem? my university lab email service has the same issue, so I guess it's a standard configuration ... ? (my mail server says it's Postfix who handled the mail, the one from the lab doesn't leak that)