Recipe to forward e-malis destined to a particular domain in qmail
1) Edit smtproutes (usually located in /var/qmail/control) and add the following line
DOMIAN_TO_FORWARD:HOST_TO_FORWARD_TO
2) Edit rcpthosts (usually located in /var/qmail/control) and add name of the domain
3) Restart qmail
Note: Make sure that the domain is not listed in the virtualdomains files too. Or qmail will process the virtualdomains file first and ignore the smtproutes entry.