[Plugin: SendPress] SPAM – INVALID_MSGID Message-Id is not valid

Hi,
eMails are classified as Spam

—
Return-Path: <xxx@xxx.com> (Adress changed)
X-Spam-Checker-Version: SpamAssassin 3.3.2 (2011-06-06) on zeus.b-connect.de
X-Spam-Flag: YES
X-Spam-Level: *****
X-Spam-Status: Yes, score=5.2 required=5.0 tests=HTML_MESSAGE,INVALID_MSGID,
MIME_HTML_MOSTLY,MPART_ALT_DIFF autolearn=no version=3.3.2
X-Spam-Report:
* 0.4 MIME_HTML_MOSTLY BODY: Multipart message mostly text/html MIME
* 0.0 HTML_MESSAGE BODY: HTML included in message
* 2.2 MPART_ALT_DIFF BODY: HTML and text parts are different
* 2.6 INVALID_MSGID Message-Id is not valid, according to RFC 2822
—–

What can i do to solve the problem?
Thanks in advance
Hardy

http://wordpress.org/extend/plugins/sendpress/