If you ever need to route mail destined for a certain domain through a smarthost you can edit/create the following file;
/var/qmail/control/smtproutes
The contents is straight forward… to relay mail destined for domain.com through smtp.yourdomain.com add the following;
domain.com:smtp.yourdomain.com
If you want to relay ALL mail through smtp.yourdomain.com add;
:smtp.yourdomain.com