disable relaying for localdomains...

hi guys,
I have enabled relaying to an SMTP authentication server, and it works
properly. Now what I want to do it to disable relaying between local
domain, i.e. if I am sending an email from a@mydomain.com to
b@mydomain.com...I want the mail not to be relayed via the SMTP
authentication....

what is the easiest way to do so?

thanks!
oumer