Go to NodeWorx >> Sytem >> Settings >> senderip (It will be set as on)
If it's off, then the IP that mail is sent from is the primary IP of the server. If that IP that has the issue (eg: Blacklisted), you can specify a different one by creating or editing /var/qmail/control/bindroutes and adding the following:
:ip.to.send.from eg: :1.0.0.0
For more details on all that, you can check this out: http://serverfault.com/questions/56801/how-can-i-configure-qmail-to-send-using-a-different-ip-per-domain
