ZF 1.5 - Zend_Mail and long subject lines

When sending an email via Zend_Mail with a long subject (about 90 characters) the whole emails gets garbled.

It seems that there is a problem with the encoding because the email body then contains parts of the email headers.
Is this intended behaviour, i guess not? Maybe i am doing something wrong, advice would be appreciated.

Thanks, ota!

Sending via Zend_Mail and stmp, utf8 encoded.

Enclosed are the first few lines without the content of the email i will recieve:

------------------------------------------------------------------------

nistrator=20viazenetti?=
Date: Mon, 17 Mar 2008 18:19:00 +0100
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Content-Disposition: inline

... encoded email content goes here ...

------------------------------------------------------------------------


The headers of the corresponding email, please note i replaced some parts because of security issues:

------------------------------------

Microsoft Mail Internet Headers Version 2.0
Received: from SERVER ([192.168.21.4]) by SERVER with Microsoft SMTPSVC(6.0.3790.3959);
         Mon, 17 Mar 2008 18:04:52 +0100
Received: from localhost (localhost [127.0.0.1])
        by SERVER (Postfix) with ESMTP id E996E8992E
        for <VALID@TO.DE>; Mon, 17 Mar 2008 18:04:43 +0100 (CET)
Message-id: <3f744eee7b8bcaec47be65eef43b6507@SERVER>
date: Mon, 17 Mar 2008 18:04:43 +0100
Sender: VALID@SENDER.DE
Errors-To: VALID@ERRORS.DE
X-Mailer: viazenetti GmbH PHP Mailer
To: "TO NAME" <VALID@TO.DE>
From: "FROM NAME" <VALID@FROM.DE>
Subject: =?UTF-8?Q?EXAMPLE=20WEBSITE=20WEBSITE=20-=20SUBJECT=20SUBJECT=20=C3=84nderung=20der=20=20-=20Administrator=
Return-Path: VALID@RETURNPATH.DE
X-OriginalArrivalTime: 17 Mar 2008 17:04:52.0437 (UTC) FILETIME=[04350050:01C88851]