This is the mail system at host cluster.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<xxx.xxx@hotmail.de>: host mailserver.burg-gmbh.de[176.9.65.175] said: 550
    Relay denied for <hotmail.de> (in reply to RCPT TO command)