Exim version 4.76 #1 built 19-Jul-2011 02:56:59
Copyright (c) University of Cambridge, 1995 - 2007
Berkeley DB: Berkeley DB 4.7.25: (November 12, 2010)
Support for: crypteq IPv6 Perl OpenSSL move_frozen_messages Content_Scanning DKIM Old_Demime
Lookups (built-in): lsearch wildlsearch nwildlsearch iplsearch cdb dbm dbmnz
Authenticators: cram_md5 plaintext
Routers: accept dnslookup ipliteral manualroute queryprogram redirect
Transports: appendfile/maildir/mailstore/mbx autoreply lmtp pipe smtp
Size of off_t: 8
Configuration file is /etc/exim.conf

[rob@server ~]$ exim -bP transports | grep dkim
dkim_canon = relaxed
dkim_domain = ${lc:${domain:$h_from:}}
dkim_private_key = ${if exists{/etc/virtual/$sender_address_domain/dkim.private.key}{/etc/virtual/$sender_address_domain/dkim.private.key}{0}}
dkim_selector = x
dkim_sign_headers =
dkim_strict = 0

# This is the mail recieved from the PHP code.
2016-11-15 08:28:52 1c6YAm-000154-6p <= mailbox@mydomain.com H=epicsoftworks.nl [ipv4.addr] P=esmtpa A=login:mailbox@mydomain.com S=22098 id=26412cc5accb22e5ce03925c7ac38a7c95c398cb19d5736fa41fb565c8dc1254@epicsoftworks.nl T="Another day at the office with DKIM..." from <mailbox@mydomain.com> for mygmail@gmail.com

# Here it is outbound for its destination. Not signed to be noted.
2016-11-15 08:28:52 1c6YAm-000154-6p => mygmail@gmail.com F=<mailbox@mydomain.com> R=lookuphost T=remote_smtp S=22157 H=gmail-smtp-in.l.google.com [ipv6.addr] X=UNKNOWN:ECDHE-RSA-AES128-GCM-SHA256:128 C="250 2.0.0 OK 1479194932 yr4si27147042wjc.210 - gsmtp"

# This is send with Thunderbird. This gets signed...
2016-11-15 08:31:47 1c6YDa-0001CM-UY => mygmail@gmail.com F=<mailbox@mydomain.com> R=lookuphost T=remote_smtp S=762 H=gmail-smtp-in.l.google.com [ipv6.addr] X=UNKNOWN:ECDHE-RSA-AES128-GCM-SHA256:128 C="250 2.0.0 OK 1479195107 s17si1915514wme.47 - gsmtp"

# This is the mail send from thunderbird. With DKIM signing.

Delivered-To: mygmail@gmail.com
Received: by 10.80.186.18 with SMTP id g18csp1289759edc;
        Mon, 14 Nov 2016 23:31:47 -0800 (PST)
X-Received: by 10.194.248.5 with SMTP id yi5mr384988wjc.11.1479195107193;
        Mon, 14 Nov 2016 23:31:47 -0800 (PST)
Return-Path: <mailbox@mydomain.com>
Received: from myserver.com (myserver.com. [ipv6.addr])
        by mx.google.com with ESMTPS id s17si1915514wme.47.2016.11.14.23.31.47
        for <mygmail@gmail.com>
        (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
        Mon, 14 Nov 2016 23:31:47 -0800 (PST)
Received-SPF: pass (google.com: domain of mailbox@mydomain.com designates ipv6.addr as permitted sender) client-ip=ipv6.addr;
Authentication-Results: mx.google.com;
       dkim=pass header.i=@mydomain.com;
       spf=pass (google.com: domain of mailbox@mydomain.com designates ipv6.addr as permitted sender) smtp.mailfrom=mailbox@mydomain.com;
       dmarc=pass (p=NONE dis=NONE) header.from=mydomain.com
DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=mydomain.com; s=x;
    h=Content-Transfer-Encoding:Content-Type:MIME-Version:Date:Message-ID:Subject:From:To; bh=zaNQl8a2eAEHfPVmKMA7RmtMqJ/6huDk4u6pr/tWrqQ=;
    b=xcDHIzzTWS8hPMxjqbZM0I6b/act/LlweTuNcnZJ9ttEF1dAm37Lzy8zOJz2E2aDTkcQOdCQuC+VyIaXTRzTMJXyzJTUXTgPUPOePsR5XYqqsE0iQRMkDl/Ah650kBHD5drqIrFJwCw5g0aL9OECqTyRO9kwL0DQJX/mKcTkLtiiIs7Z7G77ZwWhJpFm/duoQARtZZ1UZFu42/Vbl+V8vSoWbXoZBpg+WBGucWJoGq+hb5zILxwsMPcbrIu+avBjjoUdLVP9YMFiPC3nK+7zOGBWOO7x6QoHQmO8uo0P88E52Sm9ZJGgLQOCfFCMjCnv4IMemj/GSe25Sf8PKah/Xg==;
Received: from 159-032-128-083.dynamic.caiway.nl ([83.128.32.159] helo=[192.168.1.108])
    by myserver.com with esmtpsa (UNKNOWN:AES128-SHA:128)
    (Exim 4.76)
    (envelope-from <mailbox@mydomain.com>)
    id 1c6YDa-0001CM-UY
    for mygmail@gmail.com; Tue, 15 Nov 2016 08:31:46 +0100
To: Rob van der Lee <mygmail@gmail.com>
From: Rob van der Lee <mailbox@mydomain.com>
Subject: Dit is een verzonden mail via account
Message-ID: <2ccd7be7-bbd1-0fdc-a8d4-a4f6f652bfc2@mydomain.com>
Date: Tue, 15 Nov 2016 08:31:46 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101
 Thunderbird/45.4.0
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit

Dit is echt een test.

Delivered-To: mygmail@gmail.com
Received: by 10.80.186.18 with SMTP id g18csp1288906edc;
        Mon, 14 Nov 2016 23:28:52 -0800 (PST)
X-Received: by 10.28.170.134 with SMTP id t128mr2009669wme.29.1479194932632;
        Mon, 14 Nov 2016 23:28:52 -0800 (PST)
Return-Path: <mailbox@mydomain.com>
Received: from myserver.com (myserver.com. [ipv6.addr])
        by mx.google.com with ESMTPS id yr4si27147042wjc.210.2016.11.14.23.28.52
        for <mygmail@gmail.com>
        (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
        Mon, 14 Nov 2016 23:28:52 -0800 (PST)
Received-SPF: pass (google.com: domain of mailbox@mydomain.com designates ipv6.addr as permitted sender) client-ip=ipv6.addr;
Authentication-Results: mx.google.com;
       spf=pass (google.com: domain of mailbox@mydomain.com designates ipv6.addr as permitted sender) smtp.mailfrom=mailbox@mydomain.com;
       dmarc=pass (p=NONE dis=NONE) header.from=mydomain.com
Received: from mydomain.com ([37.97.128.104])
    by myserver.com with esmtpa (Exim 4.76)
    (envelope-from <mailbox@mydomain.com>)
    id 1c6YAm-000154-6p
    for mygmail@gmail.com; Tue, 15 Nov 2016 08:28:52 +0100
Date: Tue, 15 Nov 2016 07:28:52 +0000
To: mygmail@gmail.com
From: Rob van der Lee <mailbox@mydomain.com>
Sender: Rob van der Lee <mailbox@mydomain.com>
Subject: Another day at the office with DKIM...
MIME-Version: 1.0
Content-Type: multipart/alternative;
 boundary="=_7ebb8a8d12984c5cc3f5fbf995b1b4ad"
Message-ID: <26412cc5accb22e5ce03925c7ac38a7c95c398cb19d5736fa41fb565c8dc1254@mydomain.com>

This is a message in Mime Format.  If you see this, your mail reader does not support this format.

--=_7ebb8a8d12984c5cc3f5fbf995b1b4ad
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

... content of mail in text and then html, left it out since not relevant.