From where i can set this thing that perticular User's outgoing mails must be restricted to only localdomains not external one.
thanks for your help in advance.
Assuming that all your local domains are in /var/qmail/control/rcpthosts and your users don't have the ability to relay (i.e. the RELAYCLIENT environmental variable is not set), then they will only be able to send mail to those domains.
yes ... right but what about if i want to restrict few users not all the users.