Exchange server 2013 send email issue to external network

We have recently changed our ISP and we are facing the issue with exchange server sent emails.
all users are able to send internal emails and recevie emails externally too.

But when the send emails to external domain or network it gives below error.
Diagnostic information for administrators:
Generating server: MAIL.domain.com
user@domain.com
mx.domain.com
Remote Server returned '550 <user@domain.com> No such user here'
Original message headers:
Received: from MAIL.domain.com (10.0.0.8) by MAIL.domain.com
 (10.0.0.8) with Microsoft SMTP Server (TLS) id 15.0.775.38; Wed, 21 Oct 2015
 00:24:31 +0530
Received: from MAIL.domain.com ([fe80::2c25:f12b:592a:4d7c]) by
 MAIL.domain.com ([fe80::2c25:f12b:592a:4d7c%12]) with mapi id
 15.00.0775.031; Wed, 21 Oct 2015 00:24:31 +0530
From: "user1" <user1@domain.com>
To: user<user@xyz.com>
Subject: ateae
Thread-Topic: ateae
Thread-Index: AQHRC2jALYX6F1USs0yNfYYE8h1z5Q==
Date: Tue, 20 Oct 2015 18:54:30 +0000
Message-ID: <8b892d32bed240eead8351eee02d060f@MAIL.domain.com>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach:
X-MS-TNEF-Correlator:
x-originating-ip: [10.1.1.243]
Content-Type: multipart/alternative;
        boundary="_000_8b892d32bed240eead8351eee02d060fMAILrigelnetworkscom_"
MIME-Version: 1.0

For more info, we are using smart host and we have not make live our exchange server. we have configured third party tool to download  external emails.
we have configure local exchange IP port forwarding with live IP using sonicwall firewall.

We have tried to recreate send connector and restart transport service multiple times but no luck.
We can resolve our domain name with nslookup and it seems fine. but we are facing the issue with sent emails. our Production is down so quick response would be highly appreciated.

Kindly suggest...!!