In SMTP, on the 'Advanced' tab, make sure you do not have 'Transparent mode' selected. On the 'Relaying' tab, Host-based relay, 'Allowed hosts/networks', make sure you have only the host definition for your mail server, and that you do not have "Internal (Network)" included. If you have a packet filter rule that allows outbound SMTP traffic, put in a new rule in the top position: 'Internal (Network) -> SMTP -> Any : Drop'.
In SMTP, on the 'Advanced' tab, make sure you do not have 'Transparent mode' selected. On the 'Relaying' tab, Host-based relay, 'Allowed hosts/networks', make sure you have only the host definition for your mail server, and that you do not have "Internal (Network)" included. If you have a packet filter rule that allows outbound SMTP traffic, put in a new rule in the top position: 'Internal (Network) -> SMTP -> Any : Drop'.