email being bounced, but can't figure out why

I have a customer who's receiving a bounce from my server, but I can't figure out why.
We receive hundreds of emails per day.

Code:

Diagnostic information for administrators:
Generating server: xxxxxxxx.eu.corp.le
sales@mydomain.com
mydomain.com
Remote Server returned '<mydomain.com #4.4.1>'
Original message headers:
Return-Path: <S.Quest@customer.eu>
Received: from pps.filterd (xxxxxxxx.eu.corp.le [127.0.0.1])
        by leeupfp006.eu.corp.le (8.16.0.20/8.16.0.20) with SMTP id v2T7EVMK018397
        for <sales@mydomain.com>; Wed, 29 Mar 2017 09:22:49 +0200
Received: from xxxxxxxx.eu.corp.le (xxxxxxxx.eu.corp.le [172.25.xx.xx])
        by xxxxx.eu.corp.le with ESMTP id 29dfakjq7m-1
        (version=TLSv1.2 cipher=ECDHE-RSA-AES256-SHA384 bits=256 verify=NOT)
        for <sales@mydomain.com>; Wed, 29 Mar 2017 09:22:49 +0200
Received: from xxxxxxxx.eu.corp.le (172.25.16.36) by xxxxxxxx.eu.corp.le
 (172.25.16.36) with Microsoft SMTP Server (TLS) id 15.0.1178.4; Wed, 29 Mar
 2017 09:22:46 +0200
Received: from xxxxxxxx.eu.corp.le ([fe80::895d:f927:afd3:b42e]) by
 xxxxxxxx.eu.corp.le ([fe80::895d:f927:afd3:b42e%12]) with mapi id
 15.00.1178.000; Wed, 29 Mar 2017 09:22:46 +0200
From: "Quest, Sarah" <S.Quest@customer.eu>
To: "sales@mydomain.com" <sales@mydomain.com>
CC: "Derrick, Lee L." <l.derrick@customer.eu>
Subject: PO 8873996
Thread-Topic: PO 8873996
Thread-Index: AdKoXTPEZP/r7Z5aTVuqPqp0idAvDw==
Date: Wed, 29 Mar 2017 07:22:46 +0000
Message-ID: <70e2114e8ce148c0b157da820ee9ca35@xxxxxxxx.eu.corp.le>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach: yes
X-MS-TNEF-Correlator:
x-ms-exchange-transport-fromentityheader: Hosted
x-originating-ip: [xx.xxx.30.7]
Content-Type: multipart/mixed;
        boundary="_002_70e2114e8ce148c0b157da820ee9ca35xxxxxxxxeucorple_"
MIME-Version: 1.0
X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-03-29_06:,,
 signatures=0

could anyone help ?

 

In an attempt to try and resolve this quickly, I emptied my firewall block rules, and now the customer can email me.
Although I couldn't find thier IP in the block list, so maybe that was just coincidence.
We've had a number of these just lately, I've no idea whats going on.