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.
Email server behind NAT is a pain, but it can be further aggravated if your email server has a dynamic IP address. Which is the case with EC2. Granted, it will only change on stop/start of the instance, but still. Here’s a recipe to deal with it (a kludge, of course):
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: