Relay mail from a subdomain

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http://serverfault.com – I have a postfix mail server serving domain example.com. I've set up another mail server serving sub.example.com, and it relays all outgoing mail to first mail server which sends it worldwide. But the main server keep responding me with "Relay access denied." Which directive to use on the main server so it will send mails further, which are received with sub.example.com domain? (HowTos)