SOLVED 421 Refused. The domain of your sender address has no mail exchanger (MX).

This is error is generated by receiving servers using the spamdyke filtering program
Seeing the subject error in the maillog on some new CentOS 5 systems that were trying to send administrative emails. To fix it I had to change the hostname from the firstname to the FQDN.
In other words, on CentOS 5 edit /etc/sysconfig/network and put in ‘HOSTNAME=hostname.mydomain.com’ rather than just ‘HOSTNAME=hostname’