Exchange weist Mail von bestimmten Empfänger ab

Hallo liebe Comunity,

ich habe das Problem das ich nicht verstehe, warum von einem bestimmten Absender keine Mails an uns gesandt werden können.
Ich habe bereits gegoogelt aber werde leider nicht schlau daraus.
Ich hoffe das einer von euch mir da weiter helfen kann.

Wir nutzen einen Microsoft Exchange Server 2013

Anbei die Fehlermeldung die der Absender erhält.

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Diagnoseinformationen für Administratoren:
Generierender Server: mail.extern.org
vorname.name@firma.com
mail.firma.com

Remote Server returned '554 5.7.1 <mail.firma.com #5.7.1 smtp; 550-5.7.1 This email was rejected because it violates our security policy 550-5.7.1   
Inbound connection is not secured by TLS. 550 5.7.1 'Header-To' and 'CC' do not contain a local email address.>'  

Ursprüngliche Nachrichtenköpfe:

Return-Path: <vorname.name@extern.org>
Received: from utm.extern.org (mail.extern.org [213.61.189.34])
        (using TLSv1 with cipher DHE-RSA-CAMELLIA256-SHA (256/256 bits))
        (No client certificate requested)
        by mail.extern.org (Postfix) with ESMTPS id 49jyLW5p6fz4RkmT
        for <vorname.name@mail.com>; Fri, 12 Jun 2020 12:55:31 +0200 (CEST)
Received: from ex2.extern.org ([192.168.0.34]:6136)
        by utm.extern.org with esmtps (TLSv1.2:AES256-GCM-SHA384:256)
        (Exim 4.82_1-5b7a7c0-XX)
        (envelope-from <vorname.name@extern.org>)
        id 1jjhLO-000283-1u; Fri, 12 Jun 2020 12:55:30 +0200
Received: from EX1.extern.org (192.168.0.33) by EX2.zvei.org (192.168.0.34) with
 Microsoft SMTP Server (version=TLS1_2,
 cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256) id 15.1.1913.5; Fri, 12
 Jun 2020 12:55:29 +0200
Received: from EX1.extern.org ([fe80::8172:30f6:35ec:cba3]) by EX1.extern.org
 ([fe80::8172:30f6:35ec:cba3%12]) with mapi id 15.01.1913.010; Fri, 12 Jun
 2020 12:55:28 +0200
From: <vorname.name@extern.org>
To: <vorname.name@extern.org>
Subject: TA Messwandler: "Digitale Messwandler" (RM:  
 26.06.2020) / Information 
Thread-Topic: TA Messwandler: "Digitale Messwandler"  
 (RM: 26.06.2020) / Information 
Thread-Index: AdZAppwwc9Z+x4mUQ62g24SK8U+XxQ==
Date: Fri, 12 Jun 2020 10:55:28 +0000
Message-ID: <1ba21cad44854d7a9484305a24ef80d1@extern.org>
Accept-Language: de-DE, en-US
Content-Language: de-DE
X-MS-Has-Attach: yes
X-MS-TNEF-Correlator:
x-originating-ip: [192.168.16.241]
x-gfi-smtp-submission: 1
x-gfi-smtp-hellodomain: EX1.extern.org
x-gfi-smtp-remoteip: 192.168.0.33
x-exclaimer-md-config: da73dae0-1a75-41b1-8d66-34f4010fc618
Content-Type: text/plain
MIME-Version: 1.0