Make postfix/opendkim sign mail from all domains

When sending through the console or a mail client like Outlook or Mac's Mail it processes fully and has all three pass.

This behavior was expected because you place this configuration in main.cf

smtpd_milters = inet:localhost:8891

When sending through Zend Framework 1.12's SendMail function it fails (php).

This behavior was expected too because you place this configuration in main.cf

non_smtpd_milters =

Explanation

By default, OpenDKIM was combined with postfix through milter. Enabling/Disabling milter application was controlled by smtpd_milters and non_smtpd_milters parameters. For application who send email through smtpd, the correspondence parameter is smtpd_milters, for sendmail, the parameter is non_smtpd_milters

Solution

Replace non_smtpd_milters = with

non_smtpd_milters = inet:localhost:8891

Despite the fact the mail was from an external source, the accepted return path is internal. It also appears the external host used your domain in its HELO command.

Return-Path: <accounting@mydomain.com>
Received: from mydomain.com (60-241-c-d.tpgi.com.au [60.241.c.d])
    by host.mydomain.com (Postfix) with SMTP id 934533DA0157
    for <techsupport@mydomain.com>; Thu,  8 Sep 2016 14:58:48 +0000 (UTC)

The message should have been rejected based on the domain in the return path and/or the domain in the HELO command. Try to configure your server to reject mail based on these factors. I would consider accepting the message is a configuration issue for the mail server.

I believe OpenDKIM behaved correctly signing a message from a local address despite the external IP address. This would be the behavior if you had authenticated from the internet before sending.