Delivery has failed to these recipients or distribution lists

Hi everyone,

Our email system rejected the legit sender and I don't know why. Any advice would be really appreciated.

Diagnostic information for administrators:

Generating server: stssvr6.SeatoSummit.local

jteoh@hartac.com.au remote.hartac.com.au #554 5.1.0 Sender denied ##

Original message headers:

Received: from stssvr6.SeatoSummit.local ([192.168.16.106]) by

 stssvr6.SeatoSummit.local ([192.168.16.106]) with mapi; Thu, 12 Nov 2015

 12:01:37 +0800

From: Alex Kingston <alex@seatosummit.com.au>

To: Jack Teoh <jteoh@hartac.com.au>

Date: Thu, 12 Nov 2015 12:01:29 +0800

Subject: RE: Enquiry

Thread-Topic: Enquiry

Thread-Index: AdEc9lzoI5uSX+G/QAGmPVZBVKNDYAACEBJg

Message-ID: <1AD3C0DD5541B941A600E0A03033DC6BDB6026E7E0@stssvr6.SeatoSummit.local>

References: <55C2EC296A62AF458FD1D5A69DB4BCE9113A8055@HARTACDC01.hartac.local>

In-Reply-To: <55C2EC296A62AF458FD1D5A69DB4BCE9113A8055@HARTACDC01.hartac.local>

Accept-Language: en-US, en-AU

Content-Language: en-US

X-MS-Has-Attach: yes

X-MS-TNEF-Correlator:

acceptlanguage: en-US, en-AU

x-tm-as-product-ver: SMEX-8.6.0.1168-8.000.1202-21934.007

x-tm-as-result: No--39.096500-0.000000-31

x-tm-as-user-approved-sender: Yes

x-tm-as-user-blocked-sender: No

Content-Type: multipart/related;

        boundary="_008_1AD3C0DD5541B941A600E0A03033DC6BDB6026E7E0stssvr6SeatoS_";

        type="multipart/alternative"

MIME-Version: 1.0
Diagnostic information for
administrators:
Generating server:
icp-osb-irony-out8.external.iinet.net.au
jteoh@hartac.com.au
[203.23.208.115] #<[203.23.208.115] #5.0.0 smtp; 5.1.0 - Unknown address
error 554-'5.1.0 Sender denied' (delivery attempts: 0)> #SMTP#
Original message headers:
X-IronPort-Anti-Spam-Filtered: true
X-IronPort-Anti-Spam-Result: Ap4EAPSEVVXLO4c8/2dsb2JhbADTcwICAQ
X-IronPort-AV: E=Sophos;i="5.13,432,1427731200"; 
   d="gif'147?png'147,150?jpg'147,150,145?scan'147,150,145,208,217,147,150,145";a="936187440"
Received: from unknown (HELO mail.seatosummit.com.au) ([203.59.135.60])  by
 icp-osb-irony-out8.iinet.net.au with ESMTP; 15 May 2015 13:37:56 +0800
Received: from stssvr6.SeatoSummit.local ([192.168.16.106]) by
 stssvr6.SeatoSummit.local ([192.168.16.106]) with mapi; Fri, 15 May 2015
 13:37:51 +0800
From: Alex Kingston <alex@seatosummit.com.au>
To: Jack Teoh <jteoh@hartac.com.au>
Date: Fri, 15 May 2015 13:37:45 +0800
Subject: RE: Hartac
Thread-Topic: Hartac
Thread-Index: AdCOzvSSq4DK1eP6QUy5VQpcc/RqaAAAkY0A
Message-ID: <1AD3C0DD5541B941A600E0A03033DC6BB3FC233920@stssvr6.SeatoSummit.local>
References: <55C2EC296A62AF458FD1D5A69DB4BCE910DF3103@HARTACDC01.hartac.local>
In-Reply-To: <55C2EC296A62AF458FD1D5A69DB4BCE910DF3103@HARTACDC01.hartac.local>
Accept-Language: en-US, en-AU
Content-Language: en-US
X-MS-Has-Attach: yes
X-MS-TNEF-Correlator:
acceptlanguage: en-US, en-AU
x-tm-as-product-ver: SMEX-8.6.0.1168-7.500.1018-21546.004
x-tm-as-result: No--34.976800-0.000000-31
x-tm-as-user-approved-sender: Yes
x-tm-as-user-blocked-sender: No
Content-Type: text/plain
MIME-Version: 1.0

Su Myat