postfix relay internal network

[(=?utf-8?q?=60=60-=5F-=C2=B4=C2=B4?=) -- Fernando]

Hello.
Quick question:
How do set postfix to deliver email it has received from the internet, and relay it to another email server (who is the correct server for that domain inboxs) in the same lan?

-- 
BUGabundo  :o)
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ps. My emails tend to sound authority and aggressive. I'm sorry in advance. I'll try to be more assertive as time goes by...