Application Development Software

Admittedly I am a messaging server noob. Sun came in and installed our Sun JES Server for us.
That server is our LDAP, mail, and IM server.

We would like to start using gmail for domains for some of our users, but not all of them.

We use Postini for spam filtering. Right now if someone from the Internet sends an email to our domain, the mail is filtered by postini and then proceeds onward to our mail server and then local delivery to the correct mailbox.

With Postini we believe we can set it so after filtering, some email accounts can proceed to our server while others proceed onto the gmail servers.

The problem I foresee though is when sending mail internally. When we send email in house, email doesn't go through Postini and just gets delivered locally.

Is there a way in the messaging server to make it so some address continue to be delivered locally while others continue on to a remote server (i.e. either to Postini or gmail servers directly) ?