The above 553 error was probably generated on your smarthost.
Make sure your host is not called "localhost.localdomain" (in /etc/sysconfig/network). Edit /etc/hosts and make sure your host name is not in the 127.0.0.1 line (put it into separate line with your host's IP address).
thanks all - indeed the hostname was in 127.0.0.1 etc - fixed that and all is well.