My spamassassin (pretty much in default debian configuration) marks your mails as spam, for reasons that could easily be fixed.

Here's the header dump:

Received: from [removed].de (localhost.localdomain [127.0.0.1])
by [removed].de (Postfix) with ESMTP id [removed]
for <[removed]>; Wed, 2 Apr 2014 16:15:15 +0200 (CEST)
Received: by [removed].de (Postfix, from userid [removed])
id [removed]; Wed, 2 Apr 2014 16:15:15 +0200 (CEST)
X-Spam-Checker-Version: SpamAssassin 3.3.2 (2011-06-06) on [removed].de
X-Spam-Flag: YES
X-Spam-Level: *****
X-Spam-Status: Yes, score=5.0 required=5.0 tests=HTML_MESSAGE,
HTML_MIME_NO_HTML_TAG,MIME_HEADER_CTYPE_ONLY,MIME_HTML_ONLY,RDNS_NONE,
TO_NO_BRKTS_NORDNS,TO_NO_BRKTS_NORDNS_HTML autolearn=no version=3.3.2
X-Spam-Report: * 0.0 HTML_MESSAGE BODY: HTML included in message
* 1.1 MIME_HTML_ONLY BODY: Message only has text/html MIME parts
* 0.6 HTML_MIME_NO_HTML_TAG HTML-only message, but there is no HTML tag
* 1.3 RDNS_NONE Delivered to internal network by a host with no rDNS
* 2.0 MIME_HEADER_CTYPE_ONLY 'Content-Type' found without required MIME
* headers
* 0.0 TO_NO_BRKTS_NORDNS_HTML To: misformatted and no rDNS and HTML only
* 0.0 TO_NO_BRKTS_NORDNS To: misformatted and no rDNS
Received: from baka8.baka-updates.com (unknown [76.72.171.27])
by [removed].de (Postfix) with ESMTPS id [removed]
for [removed]; Wed, 2 Apr 2014 16:15:13 +0200 (CEST)
Received: from www-data by baka8.baka-updates.com with local (Exim 4.80)
(envelope-from manga@baka-updates.com)
id [removed]
for [removed]; Wed, 02 Apr 2014 07:15:09 -0700

The three HTML related things should be relatively simple to fix. (Why are you using HTML here anyway?)
I suspect that “Hotmail doesn't like us very much” is related to that…