Possible Duplicate:
How could I prevent my mail from being recognized as spam?

I'm sending a newsletter with drupals simplenews module

The website is hosted on an 1und1 server in germany (as seen in in header domains online.de and kundenserver.de)

When I send it, it goes to Spam folder in Yahoo & GMail Mailbox, but not in Spam Folder in web.de, hotmail and GMX Mailboxes

Here is, what I have in the Mail Header (for yahoo in this example)

Received: from 12.345.678.90  (EHLO sXXXXXXXXX.online.de) (12.345.678.90)
  by mtaXXX.mail.kks.yahoo.co.jp with SMTP; Fri, 15 Jun 2012 18:45:24 +0900
Received: from [127.0.0.1] (helo=infongdXXXXX.rtr.kundenserver.de)
    by sXXXXXXXXX.online.de with esmtp (Exim 4.72)
    (envelope-from <[email protected]>)
    id 1SfT5k-00068r-Q8
    for [email protected]; Fri, 15 Jun 2012 11:45:20 +0200
Received: from 83.136.130.41 (IP may be forged by CGI script)
    by infongdXXXXX.rtr.kundenserver.de with HTTP
    id 0Z04SW-1SQTKp3LPr-00YxYk; Fri, 15 Jun 2012 11:45:20 +0200
From: SENDER <[email protected]>
To: "[email protected]" <[email protected]>
Date: Fri, 15 Jun 2012 11:45:20 +0200
Subject: This is the subject of the newsletter
Thread-Topic: This is the subject of the newsletter
Thread-Index: Ac1K3nT42juzo7uCSkq5dTlby1ZvpQ==
List-Unsubscribe:
 <http://www.example.com/newsletter/confirm/remove/XXXXXXXXX>
X-MS-Has-Attach:
X-Auto-Response-Suppress: All
X-MS-TNEF-Correlator:
x-originating-ip: [12.345.678.90]
authentication-results: mtaXXX.mail.kks.yahoo.co.jp  from=example.com;
 domainkeys=neutral (no sig); dkim=neutral (no sig) [email protected]
errors-to: "SENDER" <[email protected]>
received-spf: none (sXXXXXXXXX.online.de: domain of [email protected] does
 not designate permitted sender hosts)
x-apparently-to: [email protected] via 123.45.67.890; Fri, 15 Jun 2012
 18:45:25 +0900
x-sender-info: <[email protected]>
content-length: 13762
Content-Type: multipart/alternative;
    boundary="_000_7471797868716571796675707173696675806577726778666766687_"
MIME-Version: 1.0

I cannot see any direct spam filter message in this.

But I'm kind of stunned by the Received: from 83.136.130.41 (IP may be forged by CGI script) part.

After I searched a bit, it seems, that this is a special 'feature' of 1und1 Mail servers.

Here are my questions:

• Is it possible that, if I get rid of the 'Ip maybe forged' part, that the Mail is not regarded as spam anymore?
• If so, Does anyone know, how I can get rid of it in drupal?