RCPHOSTS error

[itgroup]

Hi,
i have setup my first domain in ISPConfig and I can receive mail just fine. However, when I try to send mail (from outlook express), I receive the error message:

The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'address@server.com.au'. Subject 'Meeting Time & Sample', Account: 'sales', Server: 'mail.griffithmotorcyclecentre.com.au', Protocol: SMTP, Server Response: '553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)', Port: 25, Secure(SSL): No, Server Error: 553, Error Number: 0x800CCC79

Obviously, i suppose I have to change something in Postfix configuration somewhere? Can you please advise.
regards
steve

[falko]

Is the recipient domain listed in /etc/postfix/local-host-names?
Is the recipient email address listed in /etc/postfix/virtusertable?