1. Add domain to /var/qmail/control/rcpthosts
2. Add domain:user to /var/qmail/control/virtualdomains
3. "su -" to the user
If mail is to be stored locally:
4. Put "./Maildir/" in ".qmail-default"
If mail is to be forwarded:
4. Put forward address in ".qmail-default"
Finally, reload qmail to activate changes:
/etc/rc.d/init.d/qmail reload
For more information about how to write proper .qmail files: "man dot-qmail"