********.mail.protection.domain.com gave this error: Delivery not authorized, message refused

Some of our users have been having issues sending emails to exteal clients via MS Outlook.

Some of the time they will get a bounce back containing the following (first half of bounceback in Idea as it was considered spam);

Diagnostic information for administrators:

Generating server: email.DOMAIN.com

USER@DOMAIN ********.mail.protection.domain.com

Original message headers:

Received: from email.DOMAIN.com ([::1]) by email.DOMAIN.com ([::1]) with mapi id 14.03.0266.001; Mon, 11 Apr 2016 11:22:11 +0100 From: USER To: 'USER' Subject: RE: Boro Business Breakfast Thread-Topic: Boro Business Breakfast Thread-Index: AdGT23Vjp6diXcboSd2G08m+v3qo0f//7+kA///vF8A= Date: Mon, 11 Apr 2016 10:22:11 +0000 Message-ID: <9C550B1C1134D74D96AD23ED4FD8EF97520018AB@email.DOMAIN.com> References: <9C550B1C1134D74D96AD23ED4FD8EF975200186B@email.DOMAIN.com> In-Reply-To: Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: yes X-MS-TNEF-Correlator: x-originating-ip: [10.4.4.23] Content-Type: multipart/related; boundary="_011_9C550B1C1134D74D96AD23ED4FD8EF97520018ABemailtheadigrou_"; type="multipart/alteative" MIME-Version: 1.0

I have checked spam block lists and can't find anything against our domain/IP or their's it is more then one user that is having the issue.

Another note, it may be an un-related issue but an exteal user can't send in new emails and they can only reply to previous email threads.

So is anyone able to point me in the right direction ? I am only just getting into this side of Exchange.

Thanks in advance for any help resolving this issue.