The headers you see are not necessarily all coming from FUDforum. Your MTA may also add/change headers before the message leaves your server.

Add something like this to send_email() in iemail.inc.t and recompile your theme to log headers:

logaction(_uid, 'SEND EMAIL', 0, 'To=['. implode(',', (array)$to) .']<br />Subject=['. $subj .']<br />Headers=['. str_replace("\n", '<br />', htmlentities($header)) .']<br />Message=['. $body .']');