Atul,

There is no built-in mechanism to do that but there are a couple of work-arounds:

• Disable SMTP Auth on port 25 (primary SMTP Port) and do not allow any IP address to relay. If a local user tries sending an email out, they will get a "Relaying Denied" message. You can disable SMTP Auth by going into Server Configuration/SMTP Configuration and unchecking Allow SMTP Authentication.
• Block out-bound port 25 from your network firewall