In order to do archive scheduling, MailSteward uses the system crontab. Installing Mountain Lion deletes the /etc/crontab file that the crontab system needs in order to function. The following steps will fix the problem:
1. Open a terminal window and become the OS X system root user, by entering the 'su' command and then entering your system root user password. If you have not already enabled your system root user, here is how to do it:
touch /etc/crontabThat's all there is to it. Once this is done, you can close the terminal window. MailSteward scheduling will now work.