Users regularly send emails to users on another domain. For the past week, some work and some get bounced with 550 5.1.1 message.
I've been able to reproduce the problem sending emails to the same users. Some work some don't. Also, ALL emails are valid - I can send to via another external email.
It looks like (to me) that our exchange 2013 is rejecting some email address and not others. Heres copy of bounce back message:-
Delivery has failed to these recipients or groups:
'user1.thomas@externalcompany.gov.uk' (user1.thomas@externalcompany.gov.uk)
The email address you entered couldn't be found. Please check the recipient's email address and try to resend the message. If the problem continues, please contact your helpdesk.
Diagnostic information for administrators:
Generating server: ourexchange2013.mydomain.gov.uk
user1.thomas@externalcompany.gov.uk
Remote Server returned '550 5.1.1 RESOLVER.ADR.RecipNotFound; not found'
Original message headers:
Received: from ourexchange2013.mydomain.gov.uk (10.94.1.23) by
ourexchange2013.mydomain.gov.uk (10.94.1.23) with Microsoft SMTP Server (TLS) id
15.0.1104.5; Thu, 3 Sep 2015 11:32:52 +0100
Received: from ourexchange2013.mydomain.gov.uk ([fe80::1544:60d5:9d27:355e]) by
ourexchange2013.mydomain.gov.uk ([fe80::ed5f:a121:a3c4:4c61%16]) with mapi id
15.00.1104.000; Thu, 3 Sep 2015 11:32:51 +0100
Content-Type: application/ms-tnef; name="winmail.dat"
Content-Transfer-Encoding: binary
From: "WILLIAMS, Paul" <Paul.Williams@mycompany.gov.uk>
To: "'user1.thomas@externalcompany.gov.uk'" <user1.thomas@externalcompany.gov.uk>,
"jones,user2" <user2.jones@externalcompany.gov.uk>
Subject: RE: test email from support at abc
Thread-Topic: test email from support at abc
Thread-Index: AdDmMjYoYXi4KIC2Ra2P4rMbdBIg+wAAZbPw
Date: Thu, 3 Sep 2015 11:32:50 +0100
Message-ID: <a21fbbfe27c44d0d94c0b28cb191307a@ourexchange2013.mydomain.gov.uk>
References: <707c69d763274b00bbc53222c02f284f@ourexchange2013.mydomain.gov.uk>
In-Reply-To: <707c69d763274b00bbc53222c02f284f@ourexchange2013.mydomain.gov.uk>
Accept-Language: en-GB, en-US
Content-Language: en-US
X-MS-Has-Attach:
X-MS-TNEF-Correlator: <a21fbbfe27c44d0d94c0b28cb191307a@ourexchange2013.mydomain.gov.uk>
MIME-Version: 1.0
X-MS-Exchange-Transport-FromEntityHeader: Hosted
X-Originating-IP: [10.90.6.52]
Return-Path: Paul.Williams@mycompany.gov.uk
Only difference seems to be the way users are listed in the message above. One that works user2.jones.