Blocking mail relay from the firewall?

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.

Default port for that is 25.
If you wanna block that traffic, just drop whatever is from and to port 25. But the best way is still to configure your MTA (exim, sendmail, ...) server.