The easiest way I found was to download and configure Sendmail to relay to your SMTP server. Under Cent 5
yum install sendmail sendmail-cf
Edit /etc/mail/sendmail.mc where you find
m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
Restart the service
service sendmail restart
Check your mail log to see if and where it is sending it.