Exchange 2010 : My Email does not send to a external address

Having an issue with Exchange 2010.

I'm trying to send an email to an external recipient but when sending I instantly get the below bounceback.

This only happens to one address, sending email to any other address is fine.

I've changed the emails in the bounceback below to fictitious emails.

Requested rejected your message to the following e-mail addresses:
office@externaldomain.com (office@externaldomain.com)
Requested gave this error:
Transaction failed
A problem occurred during the delivery of this message to this e-mail address. Try sending this message again. If the problem continues, please contact your helpdesk.



Diagnostic information for administrators:
Generating server: COMPANY-EX-01.MyServer.local
office@externaldomain.com
Requested #554 Transaction failed ##
Original message headers:
Received: from COMPANY-EX-01.MyServer.local ([fe80::9cff:7ac1:270a:6af8])
 by COMPANY-EX-01.MyServer.local ([fe80::9cff:7ac1:270a:6af8%10]) with mapi
 id 14.03.0319.002; Thu, 23 Mar 2017 17:02:59 +0000
From: Jake Ives <jake@company.co.uk>
To: "office@externaldomain.com" <office@externaldomain.com>
Subject: FW: EMAIL SUBJECT
Thread-Topic: Delivery to EMAIL SUBJECT
Thread-Index: AdKi/tBvzSBmo8UoS3q0O4COzZS2BgA8xargAAFL4OAAAAvN4A==
Date: Thu, 23 Mar 2017 17:02:58 +0000
Message-ID: <5FECE51540A5FC44B6071565BC4C3D33447C967F@COMPANY-EX-01.MyServer.local>
References: <5FECE51540A5FC44B6071565BC4C3D33447C7BD8@COMPANY-EX-01.MyServer.local>
  <5FECE51540A5FC44B6071565BC4C3D33447C9654@COMPANY-EX-01.MyServer.local>
In-Reply-To: <5FECE51540A5FC44B6071565BC4C3D33447C9654@COMPANY-EX-01.MyServer.local>
Accept-Language: en-US, en-GB
Content-Language: en-US
X-MS-Has-Attach: yes
X-MS-TNEF-Correlator:
x-originating-ip: [192.168.0.32]
Content-Type: multipart/mixed;
        boundary="_004_5FECE51540A5FC44B6071565BC4C3D33447C967FMACH2EX01Mach2S_"
MIME-Version: 1.0