Question : Is my server an open relay?

My scenario:

I have a lot of postfix servers, each one, use to sent mail directly  to internet, so It's difficult to monitor them.
What I want?

Put all postfix's of my servers to send all their external mail  to an smarthost server in my network.  I mean, the smarthost must receive ONLY mail from my servers and relay them mail to internet. Remember I have a lot of different servers and domains so I don't know how to configure this smarthost becasuse in some way it's becoming an open relay.

My question:
How can I set up a secure smarthost to my network that receive ONLY from my servers and  relay all mail directly to Internet?

Answer : Is my server an open relay?

Hi,

1) normally postix trusts all networks defined in "mynetwork". And therefor "relaying mail will be allowed to all networks listed in "mynetwork".
or
2) add all postfix hosts , which relay mail to your "smarthost", to "/etc/postfix/access" on your smarthost.
then relaying for this hosts will be allowed.

Regards
Chris