HOW TO PROTECT SMTP PORT-25

I would like to restrict the SMTP PORT NO.25 (Used for outgoing mails under the domain) only for the outgoing mail server address provided by ISP and need to make sure that nobody else using the port other than the email clients under the domain.

ok, just define an internal->external firewall policy on top with source your mail server and destination all; If you don´t have restrictions above which outgoing ports your users are able to to traffic for (a questionable setup), i mean, if you don' t have these type of restrictions, inmediately below define another policy denying all outgoing smtp traffic. Then you can define your another internet policies for yous internal users.

Is there any LOG exists to find out the details of the USED PORTS.

you could define a firewall policy at bottom denying everything and logging everything.

Also kindly advise how to MONITOR all the PORTS/OPEN PORTS used in the network WITH FORTIGATE-100A.

use a network scanner; nmap from insecure.org is rock solid.

Eclipse

strange name regards

Depending on your level of experience, I' d follow up Abel and suggest giving FirePlotter a try for monitoring traffic/ports. It' s inexpensive, easy to use and doesn' t require anything other than a Windows host.