Say you have another local mailserver on your site, that will only deliver email locally
i.e. with domain gateway.myholding.com then you can this on your /var/qmail/control/smtproutes
gateway.myholding.com:
:smtp.myholding.com
then every email destined for gateway.myholding.com will be own processed
and other destination will be passed to another SMTP server (in this case smtp.myholding.com)
http://wiki.qmailtoaster.com/index.php/Smtproutes