Help!! problem with exchange server 2013

Hello ...
I beg your urgent help me I have a problem very critical emails traffic mail server 2013
I tried and checked almost everything
The problem is that occasionally people outside the organization mail proxies per user get an error message:
Delivery has failed to these recipients or groups:

hanan
A problem occurred and this message couldn't be delivered. Check to be sure the email address is correct. If the problem continues, please contact your helpdesk.








Diagnostic information for administrators:

Generating server: SEMETDC01.SEMET.local


hanan@er-s.co.il
Remote Server returned '<# 5.4.4 smtp; 554 5.4.4 SMTPSEND.DNS.NonExistentDomain; nonexistent domain>'


Original message headers:

Received: from SEMETDC01.SEMET.local (192.168.10.10) by SEMETDC01.SEMET.local
 (192.168.10.10) with Microsoft SMTP Server (TLS) id 15.0.516.32; Wed, 31 Jul
 2013 12:14:49 +0300
Received: from emea01-am1-obe.outbound.protection.outlook.com (213.199.154.14)
 by SEMETDC01.SEMET.local (192.168.10.10) with Microsoft SMTP Server (TLS) id
 15.0.516.32 via Frontend Transport; Wed, 31 Jul 2013 12:14:30 +0300
Received: from DB3PR05MB092.eurprd05.prod.outlook.com (10.255.251.149) by
 DB3PR05MB090.eurprd05.prod.outlook.com (10.255.251.145) with Microsoft SMTP
 Server (TLS) id 15.0.731.12; Wed, 31 Jul 2013 09:14:55 +0000
Received: from DB3PR05MB092.eurprd05.prod.outlook.com ([169.254.3.17]) by
 DB3PR05MB092.eurprd05.prod.outlook.com ([169.254.3.17]) with mapi id
 15.00.0731.000; Wed, 31 Jul 2013 09:14:54 +0000
From: Avi | Net-Core <avibs@net-core.co.il>
To: hanan <hanan@er-s.co.il>
Subject: sd
Thread-Topic: sd
Thread-Index: Ac6NzmK8ui + rMh + fTUaG1oo1pMm09Q ==
Date: Wed, 31 Jul 2013 09:14:53 +0000
Message-ID: <c8b9555b63264c5d81f3e624a9193af7@DB3PR05MB092.eurprd05.prod.outlook.com>
Accept-Language: en-US, en-US
Content-Language: en-US
X-MS-Has-Attach: yes
X-MS-TNEF-Correlator:
x-originating-ip: [62.90.194.59]
x-forefront-prvs: 0924C6A0D5
x-forefront-antispam-report:
Content-Type: multipart / related;
boundary = "_006_c8b9555b63264c5d81f3e624a9193af7DB3PR05MB092eurprd05pro_";
type = "multipart / alternative"
MIME-Version: 1.0
Return-Path: avibs@net-core.co.il
X-EsetResult: clean, is OK
X-EsetId: 4E99873DE419233419D5DA
After 20 minutes everything is back to his old tricks and working proper
What could be the cause ..
Importantly, except for one in a few hours it happens the server is working correctly
Thanks in advance ...
Avi Bernstein