Mail from own host is recognized as spam

[Catscrash]

Hi,

I have a problem with mail being marked as SPAM, although being
transmitted between virtual domains on the same hosts.

I checked my servers IP, it's not in any blacklist.

The X-Originating-IP is blacklisted, and I'm suspecting that that might
be the reason? But it shouldn't bother with it really, cause that's just
the DSL-IP of the sender, who used roundcube on the server.

Can you give me a tip what to do? Thanks!

Return-Path: <mail at domain1.de>
Delivered-To: mail at domain2.de
Received: from localhost (HOSTNAME.local [127.0.0.1])
	by HOSTNAME (Postfix) with ESMTP id 6A83D4210CF
	for <mail at domain2.de>; Mon, 15 Feb 2016 12:26:54 +0100 (CET)
X-Quarantine-ID: <jj4JEM7fe2eY>
X-Virus-Scanned: Debian amavisd-new at HOSTNAME
X-Spam-Flag: YES
X-Spam-Score: 6.476
X-Spam-Level: ******
X-Spam-Status: Yes, score=6.476 tagged_above=-999 required=6.31
	tests=[RCVD_IN_BRBL_LASTEXT=1.644, RCVD_IN_PBL=3.558, RDNS_NONE=1.274]
	autolearn=no autolearn_force=no
Received: from HOSTNAME ([EXTERNALIP])
	by localhost (HOSTNAME [127.0.0.1]) (amavisd-new, port 10024)
	with ESMTP id jj4JEM7fe2eY for <mail at domain2>;
	Mon, 15 Feb 2016 12:26:54 +0100 (CET)
Received: from HOSTNAME (HOSTNAME.local [127.0.0.1])
	(Authenticated sender: mail at domain1.de)
	by HOSTNAME (Postfix) with ESMTPA id DE72842091B
	for <mail at domain2.de>; Mon, 15 Feb 2016 12:26:53 +0100 (CET)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8;
 format=flowed
Content-Transfer-Encoding: 8bit
Date: Mon, 15 Feb 2016 12:26:53 +0100
From: mail at domain1.de
To: mail at domain2
Subject: ***SPAM*** Fwd: Post
In-Reply-To: <000201d167e3$49f49d60$ddddd820$@xyz.de>
References: <000201d167e3$49f49d60$ddddd820$@xyz.de>
Message-ID: <0339da7c70b287f0a730a46479e63685 at domain1.de>
X-Sender: mail at domain1.de
User-Agent: Roundcube Webmail/1.1.4
X-Originating-IP: 80.xxx.yyy.zz