LinuxQuestions.org - Postfix/Squirrelmail receives but will not send

I've Postfix on RH8. From the server box I can send and receive mail to any address (inside or out of the lan). When I connect via Squirrelmail I can still receive via other computers on the lan/wlan - but can only send to Inside addresses (within the lan).

Squirrelmail returns

Quote:

Transaction failed
Server replied: 554 <anyone@somewhere.com>: Recipient address rejected: Relay access denied

I feel that I've missed some permission somewhere in my main.cf file (Postfix), but I can't find were.

Any Ideas? Thanks,
kay