Solved Cron Emails & OpenDKIM?

Is there any reason why cron emails are not signed by OpenDKIM?

My server is setup so all emails are delivered via postfix (sendmail is disabled). The cron emails are delivered from <user> If I manually send an email from "", the email is digitally signed by DKIM.

However, the cron emails (using the same email address) do not get signed. I checked the maillog and confirmed Postfix is delivering these cron emails.
postfix/qmgr[88818]: CAD0B4595F0: from=<>, size=2773, nrcpt=1 (queue active)
Any ideas where to start troubleshooting this?
Do you have Syslog and Logwhy enabled in opendkim.conf? If you're missing entries corresponding to the emails, postfix is skipping opendkim entirely as opposed to opendkim not signing.
Nothing shows up in /var/log/maillog with Logwhy enabled.

In fact, there is nothing to indicate that DKIM was used/not used when sending emails. The only entries its shows is when it's started/restarted.
How is it configured in postfix? There is an smtpd_milters and non_smtpd_milters. I'm guessing you don't have the second one? That's used for submission via the local sendmail binary whereas the first option is via smtp.
I did not have non_smtpd_milters, which solved the issue. Thank you both very much for all your help!