customer mails are marked as spam

[MG_Sky_Georg]

Hi everybody, i have, again, a problem with the Plesk-Mailing-System...

On my main Domain the Mailserver works fine, but the mail's of the customers with a different domain get lost inside the spamfolders of gmail and hotmail :-(

i have , simply no idea.

Mailheader of a working adress:

-----
Delivered-To: mgskytest@gmail.com
Received: by 10.37.209.201 with SMTP id i192csp2242492ybg;
Tue, 9 Feb 2016 14:24:08 -0800 (PST)
X-Received: by 10.28.102.68 with SMTP id a65mr7756893wmc.12.1455056648632;
Tue, 09 Feb 2016 14:24:08 -0800 (PST)
Return-Path: <ama@mg-sky.de>
Received: from mg-sky.de (mg-sky.de. [85.25.197.226])
by mx.google.com with ESMTPS id t8si269633wje.33.2016.02.09.14.24.08
for <mgskytest@gmail.com>
(version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
Tue, 09 Feb 2016 14:24:08 -0800 (PST)
Received-SPF: pass (google.com: domain of ama@mg-sky.de designates 85.25.197.226 as permitted sender) client-ip=85.25.197.226;
Authentication-Results: mx.google.com;
spf=pass (google.com: domain of ama@mg-sky.de designates 85.25.197.226 as permitted sender) smtp.mailfrom=ama@mg-sky.de
Received: from mg-sky.de (localhost [127.0.0.1])
by mg-sky.de (Postfix) with ESMTPSA id 891F719C2A16
for <mgskytest@gmail.com>; Tue, 9 Feb 2016 23:24:07 +0100 (CET)
DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws;
s=default; d=mg-sky.de;
b=iUbTk2O7ew4uF1/mJ6ql0nQBONAXIDP2zr2JboHrC0r1nvX+Rik34nwmmRX6rAHi2nWpBeruGUAgj99vwD1l9emuCwRUZQHNIHrbDd/RlKPHyq8u2EuVll4YSeV9p4u/Y4ND/YyDGKt8nqy8a8vXCSV69subHhNLPunq0LFlcxA=;
h=To:Subjectate:Message-ID:MIME-Version:Content-Type:X-Mailer:Thread-Index:Content-Language:User-Agent:From:X-PPP-Message-ID:X-PPP-Vhost;
To: mgskytest@gmail.com
----

and one emailheader of a not working domain:
----
Delivered-To: mgskytest@gmail.com
Received: by 10.37.209.201 with SMTP id i192csp2243595ybg;
Tue, 9 Feb 2016 14:27:13 -0800 (PST)
X-Received: by 10.194.19.138 with SMTP id f10mr35915790wje.108.1455056833233;
Tue, 09 Feb 2016 14:27:13 -0800 (PST)
Return-Path: <ama@car-point-allen.de>
Received: from car-point-allen.de (mg-sky.de. [85.25.197.226])
by mx.google.com with ESMTPS id xb9si271386wjc.63.2016.02.09.14.27.13
for <mgskytest@gmail.com>
(version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
Tue, 09 Feb 2016 14:27:13 -0800 (PST)
Received-SPF: pass (google.com: domain of ama@car-point-allen.de designates 85.25.197.226 as permitted sender) client-ip=85.25.197.226;
Authentication-Results: mx.google.com;
spf=pass (google.com: domain of ama@car-point-allen.de designates 85.25.197.226 as permitted sender) smtp.mailfrom=ama@car-point-allen.de
Received: from mg-sky.de (localhost [127.0.0.1])
by mg-sky.de (Postfix) with ESMTPSA id DE80219C2A37
for <mgskytest@gmail.com>; Tue, 9 Feb 2016 23:27:11 +0100 (CET)
DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws;
s=default; d=car-point-allen.de;
b=K+u04dcif8b4qJybaVBs7Yrhzq8k/DVgylMaFWOsUI27OZpa3y4M7NxTcHDs+WSt8UYW/Cn9bQ4ptSppaFcSj4OhBEaMJhQbTLqZnbK5AsNM/FINNWjgoOid4M/9p7kj9xW4gmRsXCp6WeMFcia/5QVeyYVhHMgRa3TJ+D/p8Pw=;
h=Receivedate:Message-ID:From:To:Subject:User-Agent:Content-Type:MIME-Version:Content-Disposition:X-PPP-Message-ID:X-PPP-Vhost;
---

do you have any idea?

i have none :'(

Greetings