Make postfix send emails from server dedicated IP


I have a virtualmin installation with many virtual servers, and many IPs.
Some IPs are dedicated to some virtual servers.

Let assume IPs AAA.AAA.AAA.AAA and BBB.BBB.BBB.BBB are shared IPs,
and CCC.CCC.CCC.CCC are a IP dedicated to one virtual server.

I need postfix to send emails for the virtual server with the dedicated IP CCC.CCC.CCC.CCC from that IP, not from the default shared IP AAA.AAA.AAA.AAA as it do now.

Could you tell me how to configure this ?