using the postfix mail server as smtp output without relaying

I have a local network here and a web/mail host on line mail.thisdomain.tld

On the lan, pc1 is connected through adsl and is the lan gateway for the other machines
I have a mac os x (Mac1) connected to the gateway but when i use it to send message to the postfix list (for example) using mail.thisdomain.tld as smtp server, the message is rejected : 554 relaying denied

Same thing if i use the gateway as mail client.

pas glop;

What should i do ?

Pascal



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