+# Milter configuration
+# Based on https://www.linode.com/docs/guides/configure-spf-and-dkim-in-postfix-on-debian-9/#testing-postfix-dkim-on-debian-9-with-a-gmail-test
+# http://www.opendkim.org/opendkim-README
+# http://www.postfix.org/MILTER_README.html
+# OpenDKIM
+milter_default_action = accept
+# Postfix ≥ 2.6 milter_protocol = 6, Postfix ≤ 2.5 milter_protocol = 2
+milter_protocol = 6
+smtpd_milters = unix:opendkim/opendkim.sock
+non_smtpd_milters = $smtpd_milters
+