share does not work between users on same email provider

I believe this might be a bug so this has been converted to bug 609999

problem still exists.

I create a folder and want to share it with my wife.
my email r.nnnn.gmx.de (changed a little for privacy)
other email <email address hidden> (changed a little for privacy)
as we both use same email provider, the email with the share information NEVER reaches my wife.
I get an ERROR message:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

  <email address hidden>
    SMTP error from remote mail server after RCPT TO:<email address hidden>:
    host mx0.gmx.net [213.165.64.100]: 550-5.7.1 {mx023} The recipient does not accept mails from 'gmx.de' over foreign mailservers
    550 5.7.1 ( http://portal.gmx.net/serverrules )

------ This is a copy of the message, including all the headers. ------

Return-path: <email address hidden>
Received: from naranjilla.canonical.com ([91.189.89.66])
    by adelie.canonical.com with esmtp (Exim 4.69 #1 (Debian))
    id 1PAHtr-0005nq-2o
    for <email address hidden>; Mon, 25 Oct 2010 08:55:23 +0100
Received: from naranjilla.canonical.com (localhost [127.0.0.1])
    by naranjilla.canonical.com (Postfix) with ESMTP id 0496715A083
    for <email address hidden>; Mon, 25 Oct 2010 07:55:23 +0000 (UTC)
Content-Type: text/plain; charset="utf-8"
MIME-Version: 1.0
Content-Transfer-Encoding: quoted-printable
Subject: None Richard shared test_share_richard with you via Ubuntu One
From: <email address hidden>
To: <email address hidden>
Date: Mon, 25 Oct 2010 07:55:23 -0000
Message-ID: <email address hidden>

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D