If still want to go with this way (rebooting the server every day at 17:00), then:
1. check where is located your reboot command (an whereis reboot should be enough)
2. change the crontab specifying the full path and add --force to the command, like:
/sbin/reboot -f
On the other hand, in order to find out the root cause of this behavior you can try to follow the steps provided in this Postfix Debugging Howto Opens a new window, maybe it helps.
