As you might know, cron captures all the output of executed tasks and mails them to the user under which the tasks are executed. The problem is that often this mail just piles up somewhere in /var/mail directory, without being ever reviewed. It’s not a good practice, akin to sweeping the trash under the carpet.
Centos 7 has Exim 4.84 in repos, which has DKIM support built-in. So no need for OpenDKIM anymore. See how is it configured: