Email sent from my postfix server is flagged as spam in gmail

Question

Like the title says is my mail flagged as spam in gmail. I do have ideas that it has something to do with the 127.0.0.1 part but i'm not sure. The header looks like this:

Delivered-To: xxxxxx@gmail.com
Received: by 10.79.116.196 with SMTP id z65csp804720ivf;
        Tue, 4 Jul 2017 05:52:23 -0700 (PDT)
X-Received: by 10.28.166.132 with SMTP id p126mr19366025wme.104.1499172743690;
        Tue, 04 Jul 2017 05:52:23 -0700 (PDT)
ARC-Seal: i=1; a=rsa-sha256; t=1499172743; cv=none;
        d=google.com; s=arc-20160816;
        b=JUNkKoCHdTFQG0pcj/wWyabfbFyTg3RAmlzfkJX9Qj1I6SAp0ax/RBkGh3tIH9TsJN
         5S2h46c+Fw5olZ0O4h7oYTeRo1u9OBwrgnTR5AjoirzWlMiB2JrsZgAnBa0z/XbKM5G/
         fo6BnTkVuBcy3g8fQMMQ/SfRlY9w9Yar8DumN7FJDvVWfyGcllPBA0EPB0TatyRwozZH
         DoeWG8vcdOsygXwNhvVA9CrPzUuRNxq4I5G+gp/e7SHFJxaFA8tgdcktVof1k00xMYN8
         td8ztlizq4btKX1GJcPj3mnw0IhqHa1IdWw6ltNFcR3lfEkz2xEQQmuNHDBsVA44oTuf
         loCQ==
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816;
        h=message-id:sender:errors-to:cc:reply-to:from:list-subscribe
         :list-help:list-post:list-archive:list-unsubscribe:list-id
         :precedence:subject:importance:date:to:mime-version:delivered-to
         :dkim-signature:arc-authentication-results;
        bh=1hE9+wnHb3d/rgd9qD7TH7Jz1JGadEb6Ysgwu49/Y6E=;
        b=LRxkUGftBv98uDvJUkr2/POrakzx1HIRsIrwhC21fGdRGhrc6p//UR6GTvy045b6Df
         wYBunSEGtWDYs/oJiI8X+L0rM60Ic25uzkidai8N4gK/hRf1SvBRKd7CBbY9+zYexzwF
         G06kL/jazj9U2CcYA+H5U/dJbnkX9806uxBTtQ8j/W9OQbZDwfnsdKJMLvm1u8aePo4Z
         E9zPqgNxowxmkhRNVulC5tDv4bkiNjB9LveDNZAaYh6J38G6QC/DivaUKlsUAH8GUA52
         mymT6Saas1C/Ec8X0MZbb7Y1BMhpggKc5JLAZh/nEpI/h7/fx+AxfUojejuGCNu9TqTm
         sLjw==
ARC-Authentication-Results: i=1; mx.google.com;
       dkim=pass (test mode) header.i=@sub.mydomain.com header.b=5pztakr1;
       spf=pass (google.com: domain of oc-bounces@sub.mydomain.com designates 5.196.4.39 as permitted sender) smtp.mailfrom=oc-bounces@sub.mydomain.com
Return-Path: <oc-bounces@sub.mydomain.com>
Received: from sub.mydomain.com (39.ip-5-196-4.eu. [5.196.4.39])
        by mx.google.com with ESMTP id k5si40036wma.87.2017.07.04.05.52.23;
        Tue, 04 Jul 2017 05:52:23 -0700 (PDT)
Received-SPF: pass (google.com: domain of oc-bounces@sub.mydomain.com designates 5.196.4.39 as permitted sender) client-ip=5.196.4.39;
Authentication-Results: mx.google.com;
       dkim=pass (test mode) header.i=@sub.mydomain.com header.b=5pztakr1;
       spf=pass (google.com: domain of oc-bounces@sub.mydomain.com designates 5.196.4.39 as permitted sender) smtp.mailfrom=oc-bounces@sub.mydomain.com
Received: from sub.mydomain.com (sub.mydomain.com [127.0.0.1]) by sub.mydomain.com (Postfix) with ESMTP id 7901544614; Tue,
  4 Jul 2017 14:52:22 +0200 (CEST)
DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=sub.mydomain.com; s=mail; t=1499172742; bh=HDoFwCVNCHgLH68W6KRlIndQHwDMy4I9S5ct8b8JNCQ=; h=To:Date:Subject:List-Id:List-Unsubscribe:List-Archive:List-Post:
     List-Help:List-Subscribe:From:Reply-To:Cc:From; b=5pztakr1swI9StkND4XExkmxLPV7fW+jluXuq6BDgcsQoF5Jkw4w86nzMe5eXe4qh
     88VZy4P4Frb682bqHgQ6MF8lU90FnmHVu4Q4ARN/B50szZZswnm4WpWH4xThNNtDnm
     7rgUSQsJMn6VJAul0lyFHhdAL5LsyZS20Wzmfoxw=
X-Original-To: oc@sub.mydomain.com
Delivered-To: oc@sub.mydomain.com
Received: from s184.webhostingserver.nl (h2-sp01.webhostingserver.nl [141.138.168.55]) by sub.mydomain.com (Postfix) with ESMTPS id C31D244614 for <oc@sub.mydomain.com>; Tue,
  4 Jul 2017 14:52:19 +0200 (CEST)
Received: from 5070903e.static.ziggozakelijk.nl ([80.112.144.62] helo=[IPv6:::ffff:192.168.1.4]) by s184.webhostingserver.nl with esmtpa (Exim 4.87) (envelope-from <mart@somedomain.com>) id 1dSNJT-001wgG-QK for oc@sub.mydomain.com; Tue, 04 Jul 2017 14:52:19 +0200
MIME-Version: 1.0
To: "oc@sub.mydomain.com" <oc@sub.mydomain.com>
Date: Tue, 4 Jul 2017 14:52:16 +0200
Importance: normal
X-Priority: 3
Content-Type: multipart/alternative; boundary="_F4AC0B88-6980-434B-B17E-6D03F508243E_"
X-Authenticated-Id: mart@somedomain.com
Subject: [OC] Mail test
X-BeenThere: oc@sub.mydomain.com
X-Mailman-Version: 2.1.23
Precedence: list
List-Id: Opleidings Commissie <oc.sub.mydomain.com>
List-Unsubscribe: <http://mail.sub.mydomain.com/mailman/options/oc>, <mailto:oc-request@sub.mydomain.com?subject=unsubscribe>
List-Archive: <http://mail.sub.mydomain.com/mailman/private/oc/>
List-Post: <mailto:oc@sub.mydomain.com>
List-Help: <mailto:oc-request@sub.mydomain.com?subject=help>
List-Subscribe: <http://mail.sub.mydomain.com/mailman/listinfo/oc>, <mailto:oc-request@sub.mydomain.com?subject=subscribe>
From: mart--- via OC <oc@sub.mydomain.com>
Reply-To: Opleidings Commissie <oc@sub.mydomain.com>
Cc: mart@somedomain.com
Errors-To: oc-bounces@sub.mydomain.com
Sender: OC <oc-bounces@sub.mydomain.com>

GMAIL says: It's similar to messages that were detected by our spam filters. Though is this the real issue?