Setup postfix

Install postfix

yum install postfix

Remove sendmail

/etc/init.d/sendmail stop
yum remove sendmail sendmail-cf

Start postfix

/etc/init.d/postfix start
chkconfig postfix on

Configuration

vim /etc/postfix/main.cf
myhostname = www.my-helper.com
myorigin = $myhostname

Restart postfix

/etc/init.d/postfix check
/etc/init.d/postfix restart
netstat -tlunp | grep :25

reference: http://linux.vbird.org/linux_server/0380mail.php