Restricting port access to SMTP

Hey guys

I want to connect my Fortimail 200E appliance directly to the internet on port 1, by giving it an external IP addresses directly from my ISP subnet.

Is there a way to restrict traffic on this port(port 1) only to SMTP traffic.

I see in the the port configuration we can restrict it to http, https, ping, SNMP, SSH and TELNET. 

I was wondering if there would be a way to restrict access on this external port only to SMTP traffic.

-thanks