Help! Need to delete outgoing mail queue

postsuper will work fine, but you don't need that long command line. postsuper is in /usr/sbin, which is in your PATH, so simply:
sudo postsuper -d ALL (ALL has to be uppercase).

If you want to try it first on a single message, run mailq, which will scroll all of those queued messages across the screen. Copy the Queue ID of the last message off the screen, and enter:
sudo postsuper -d queue_id
then run mailq again, and that last message should be gone.

Before deleting the entire queue, make sure you don't have any legitimate pending mail.