mail routing

[chino malasugi adnu edu ph]

Hi,

I'm using Redhat 6.0.  I'm trying to configure my server such that when
a mail sender sends an email to 

	user adnu edu ph, 

it will try to send the mail first to 

	user sili adnu edu ph

if the user does not exist in     sili.adnu.edu.ph, then
it will automatically send the mail to 

	user pili adnu edu ph

assuming  the user exist in    pili.adnu.edu.ph.

Anybody could help me solve this.  thanks a lot

carlo aureus