That's not a bug... "X-To:" indicate the list of OTHER recipients of the mail
file, ie all the people which appear on the "To:" field, EXCEPT the list itself
(which necessarily  was a  recipient of  the mailing)  AND the  originator (the
latter being  because I sometimes  have to cc: myself  in my mailing  cause the
recipient is  not in my  NAMES file and that  $!"%!$+ MAIL then  says "Outgoing
mail was  not logged"). If  there is no other  recipient, the "X-To:"  field is
empty and does not appear in the header. The same applies to "X-cc:".
 
  Eric