Blocking mail relay from the firewall?

["Reginald NALLARATNAM" <regi@xxxxxxxxxxxxxxxx>]

Hello,

  I hava a Linux box private ip @ 192.168.3.1 which acts as a firewall.   The SMTP mail server behind the firewall @ 192.168.3.10.
All mail trafic is beeing forwarded to mail server from the firewall.   How can I stop mail relay from the Linux firewall?

Thanks in advance.