email being rejected by zoho server

I have one client who can not send me emails. They are rejected with the following information. The email address is correct. I also get emails from other hhangus.com addresses.

From: Mail Delivery System [mailto:MAILER-DAEMON@LOCALHOST]

Sent: October 05, 2017 12:23 PM
To: Craig Sievenpiper

Subject: Undeliverable: RBC GCC End of Life Refresh

Diagnostic information for administrators:

Generating server: mail.hhangus.com

jack.koch@4dcsolutions.com
Remote Server returned '< #4.4.3 X-Email-Security-Gateway; Host or domain name not found. Name service error for name=4dcsolutions.com type=MX: Host not found, try again>'

Original message headers:

Return-Path: <Craig.Sievenpiper@hhangus.com>

X-ASG-Debug-ID: 1507036628-0dd8285562d4d40001-MhL2Wu

Received: from HHAExchange2.HHA.AngusGroup.local ([172.18.1.33]) by mail.hhangus.com with ESMTP id 4IZvZBVHJxwFWuET (version=TLSv1.2 cipher=ECDHE-RSA-AES256-SHA384 bits=256 verify=NO); Tue, 03 Oct 2017 09:17:08 -0400 (EDT)

X-Barracuda-Envelope-From: Craig.Sievenpiper@hhangus.com

Received: from HHAExchange2.HHA.AngusGroup.local (172.18.1.33) by

 HHAExchange2.HHA.AngusGroup.local (172.18.1..33) with Microsoft SMTP Server

 (TLS) id 15.0.1178.4; Tue, 3 Oct 2017 09:17:08 -0400

Received: from HHAExchange2.HHA.AngusGroup.local ([fe80::69cc:d940:de7c:7680])

 by HHAExchange2.HHA.AngusGroup.local ([fe80::69cc:d940:de7c:7680%12]) with

 mapi id 15.00.1178.000; Tue, 3 Oct 2017 09:17:08 -0400

From: Craig Sievenpiper <Craig.Sievenpiper@hhangus.com>

To: "Jack Koch (jack.koch@4dcsolutions.com)" <jack.koch@4dcsolutions.com>,

        "Matt Plato (mattplato@yahoo.com)" <mattplato@yahoo.com>

Subject: FW: RBC GCC End of Life Refresh

Thread-Topic: RBC GCC End of Life Refresh

X-ASG-Orig-Subj: FW: RBC GCC End of Life Refresh

Thread-Index: AQHTMJpcc+4uZ94RDkirhyzOlgrVcqLSMhYA

Date: Tue, 3 Oct 2017 13:16:58 +0000

Deferred-Delivery: Tue, 3 Oct 2017 13:16:22 +0000

Message-ID: <05a3a1a6a1df4ab085862e9ef1345c4b@HHAExchange2.HHA.AngusGroup.local>

Accept-Language: en-CA, en-US

Content-Language: en-US

X-MS-Has-Attach:

X-MS-TNEF-Correlator:

x-ms-exchange-transport-fromentityheader: Hosted

x-originating-ip: [172.18.9.56]

Content-Type: multipart/alternative;

        boundary="_000_05a3a1a6a1df4ab085862e9ef1345c4bHHAExchange2HHAAngusGro_"

MIME-Version: 1.0

X-Barracuda-Connect: UNKNOWN[172.18.1.33]

X-Barracuda-Start-Time: 1507036628

X-Barracuda-Encrypted: ECDHE-RSA-AES256-SHA384

X-Barracuda-URL: https://mail.hhangus.com:443/cgi-mod/mark.cgi

X-Virus-Scanned: by bsmtpd at hhangus.com

X-Barracuda-Scan-Msg-Size: 13906

X-Barracuda-BRTS-Status: 1

X-Barracuda-Bayes: INNOCENT GLOBAL 0.4991 1.0000 0.0000

X-Barracuda-Spam-Score: 0.00

X-Barracuda-Spam-Status: No, SCORE=0.00 using global scores of TAG_LEVEL=1000.0 QUARANTINE_LEVEL=1000.0 KILL_LEVEL=6.0 tests=HTML_MESSAGE

X-Barracuda-Spam-Report: Code version 3.2, rules version 3.2.3.43556

        Rule breakdown below

         pts rule name              description

        ---- ---------------------- --------------------------------------------------

        0..00 HTML_MESSAGE           BODY: HTML included in message