Relay 550 wenn Mail an Bekannten

Versand an bestimmte Mail-Adresse kommt immer mit Fehler 550 zurück.

Hallo zusammen,

ein Bekannter hat mich angerufen und mir gesagt er bekomme keine Mails mehr von seinen Kunden. Ich habe daraufhin mal getestet ob meine Mail ankommt und folgendes zurückbekommen, könnt Ihr mir hier weiterhelfen?

Sorry, we were unable to deliver your message to the following address.

<abc@xyz.de>:
Remote host said: 550 won't relay to abc@xyz.de [RCPT_TO]

--- Below this line is a copy of the message.

Received: from [217.12.4.215] by n9.bullet.ukl.yahoo.com with NNFMP; 21 Nov 2008 12:36:32 -0000
Received: from [87.248.110.113] by t2.bullet.ukl.yahoo.com with NNFMP; 21 Nov 2008 12:36:31 -0000
Received: from [127.0.0.1] by omp218.mail.ukl.yahoo.com with NNFMP; 21 Nov 2008 12:36:32 -0000
X-Yahoo-Newman-Id: 19793.65416.bm@omp218.mail.ukl.yahoo.com
Received: (qmail 79626 invoked from network); 21 Nov 2008 12:36:31 -0000
DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws;
s=s1024; d=yahoo.de;
h=Received:X-YMail-OSG:X-Yahoo-Newman-Property:Message-ID:From:To:Subject:Date:MIME-Version:Content-Type:X-Priority:X-MSMail-Priority:X-Mailer:Disposition-Notification-To:X-MimeOLE;
b=GaNUWHDHhICcPTbu8IK8RnqHYM79Wyckiji6gu6qq5QRjGi800aNefsdiVVp7TSs1IZejX/t0mY38/6vkyI3S4VDULN2EE27gZPrMcCCmhV4UzjTlIUEOJUob/hMVj0weS4BXDHAcOoNSfCpxj1loIgQyf84WxjRPe48TYwwv7U= ;
Received: from unknown (HELO RainerMobil) (cbacba@84.149.240.152 with login)
by smtp109.mail.ukl.yahoo.com with SMTP; 21 Nov 2008 12:36:31 -0000
X-YMail-OSG: Io1hkwwVM1nFx9vQmXOMVGsIegR_.6YJyHQ1KKA_hqgcUHUvghwetxsVa8BGxJHEpA1Nq5D16oPINj1RCj4Yvy0dUPe7UriGwON_1c7nPlkDW2XLiMhw6aXAk3qpJmEgEt3MOEGQ2tqzdldvoB94QuT4I.3Ns6UFZdDsTrg-
X-Yahoo-Newman-Property: ymail-3

<abc@xyz.de>:
Remote host said: 550 won't relay to abc@xyz.de [RCPT_TO]