I'd think so.
Not sure what is causing this to happen. Perhaps mysql is interpreting teh datestring as a string..? (Sounds too weird to be true though).
What you could do.. Instead of having the date field: DATE_FORMAT( message_date, '%m/%d/%y' ) AS message_date
try to use: DATE_FORMAT( message_date, '%m/%d/%y' ) AS message_out_date
and keep the order by statement as is. Then you are actually sorting by the timestamp in the database.