CentOS 6 as smarthost (sendmail)
yum install mailx sendmail sendmail-cf -y
vi /etc/mail/sendmail.mc
dnl define(`SMART_HOST', `smtp.domain.com')dnl
dnl MASQUERADE_AS(`domain.com')dnl
m4 /etc/mail/senmdmail.mc > /etc/mail/sendmail.cf
chkconfig sendmail on
service sendmail restart