Cannot send mail to specific domain

Hi Guys

We're using Exchange 2013 and we're having a problem where we're completely unable to send any mails to 1 specific domain. We get bounce backs saying unable to deliver but will keep trying, and eventually get:

Received: from REMOVED by REMOVED

(10.0.0.28) with Microsoft SMTP Server (TLS) id 15.0.1104.5; Wed, 9 Dec 2015

14:33:14 +0000

Received: from REMOVED ([::1]) by REMOVED ([::1])

with mapi id 15.00.1104.000; Wed, 9 Dec 2015 14:33:14 +0000

From: Sender

To: Recipient

Subject: RE: Blocking

Thread-Topic: Blocking

Thread-Index: AdEyjlFXWf+hfTFkR16j7SdFhr2+0AAADMhQ

Date: Wed, 9 Dec 2015 14:33:13 +0000

Message-ID: REMOVED

References: REMOVED

In-Reply-To: REMOVED

Accept-Language: en-GB, en-US

Content-Language: en-US

X-MS-Has-Attach: yes

X-MS-TNEF-Correlator:

x-ms-exchange-transport-fromentityheader: Hosted

x-originating-ip: [10.0.1.113]

Content-Type: multipart/related;

        boundary=REMOVED

        type="multipart/alternative"

MIME-Version: 1.0
When checking Exchange Queue Viewer the message in the queue has the error

Last Error: 451 4.7.0 Recipient rejected (R20)

I've removed any identify data from the above.

Have been in touch with the IT guys of the company we're trying to contact and they assure us that they can't see any issues on their side. We don't have any issues sending mail to any other domains.

Do you have any ideas on what may be causing this issue, let me know what other info you need and i will try to provide it.

Thanks