Check the logs. Qmail logs any transactions into /var/log/qmail/qmail-send. Go into that directory, and grep for the domain you are sending from. See if your machine is receiving it and can't deliver to the user (you should create users through the qmailadmin interface, which qmailrocks puts at
http://yourserver.com/cgi-bin/qmailadmin), or if it is not even receiving it. If you don't receive it, the problem is DNS. If your server gets the message but can't deliver to the user, see what went wrong with the creation of that particular user.
Peace,
JimBass