A Redhat package problem caused CentOS 5 machines (Linux Servers) to loose access to certain network services. One of our two mail servers may have rejected email between 04:15 and 04:30, when the problem was spotted and the failing MTA was taken out of service.
A script was run to fix all effected servers, but any which were down at the time will not have been fixed. User admins can fix it using:
cl-asuser restorecon -v /etc/nsswitch.conf
which should report that it fixed the file, and then reboot to fix things. Alternatively, email sys-admin with the name of the machine.