Disable Sending Emails Using Iptables

#locate qmail 
/usr/share/logwatch/scripts/services/qmail 
/usr/share/logwatch/scripts/services/qmail-pop3d 
/usr/share/logwatch/scripts/services/qmail-pop3ds 
/usr/share/logwatch/scripts/services/qmail-send 
/usr/share/logwatch/scripts/services/qmail-smtpd 

iptables -A OUTPUT -p tcp –-dport 25 -j REJECT 

/sbin/service iptables save