If you see the error ( mailman has failed. Contact your system administrator if the service does not automagically recover. ) Here are the simple steps to resolve this. If Mailman is unable to start and you cannot find another reason for this, it may be that some of the configuration files are corrupted. So the easiest way to resolve is to simply reinstall the Mailman RPM.
rpm -e --nodeps cpanel-mailman
/usr/local/cpanel/scripts/check_cpanel_rpms --fix --targets=mailman
