It's not entirely clear what you want to do here. Do you only want to route mail sent internally to internal users out to Postini, or do you want to route all mail sent
internally without regard to who it is sent to?
Assuming it's the former, my suggestion would be to use the aliasdetourhost channel keyword. See:
for a description of how it works.
Basically what you'd do is create a tcp_postini channel to route mail to Postini. I'd recommend using the daemon keyword to specify what host/domain Postini tells you to use for this purpose. You'll need to mark it with the dequeueremoveroute keyword as well. Something like this:
tcp_postini smtp dequeueremoveroute daemon xxx.postini.com
You will need to create a special domain name to route to this channel, say postini.domain.com. Create a rewrite rule:
Now place the aliasdetourhost keyword on whatever channel or channels internal mail originates from with postini.domain.com as its argument. (If you don't have internal mail segregated this way you'll need to set that up - there has to be a way to tell internal from external in order to route internal differently.)
This will cause any mail sent to an address that's seen as internal to be rerouted to the tcp_postini channel and out to Postini.
As I said, I'm assuming things here that may not be valid, so this may or may not be appropriate for your purposes.