De resolutions in this case was setting up and add a spf record to the DNS records at my hostingcompany. I still don't know what this does actually but it solved my problem.
Maybe it is to someone's use......
More info: http://openspf.org
Reporting-MTA: dns; vhosting-relay.mail.pcextreme.nl
X-Postfix-Queue-ID: D1D26415B6
X-Postfix-Sender: rfc822; admin@mysite.nl
Arrival-Date: Mon, 17 May 2010 14:27:40 +0200 (CEST)
Final-Recipient: rfc822; address@hotmail.com
Original-Recipient: rfc822;address@hotmail.com
Action: failed
Status: 5.0.0
Remote-MTA: dns; mx4.hotmail.com
Diagnostic-Code: smtp; 550 SC-001 Mail rejected by Windows Live Hotmail for
policy reasons. Reasons for rejection may be related to content with
spam-like characteristics or IP/domain reputation problems. If you are not
an email/network admin please contact your E-mail/Internet Service Provider
for help. Email/network admins, please visit http://postmaster.live.com for
email delivery information and support
From: =?utf-8?B?SlpEIEhlbGlvcw==?=
Subject: Testmail
Message-ID:
X-Priority: 3
X-Mailer: PHPMailer (phpmailer.sourceforge.net) [version 2.0.0 rc3]
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset="utf-8"