Strange Undeliverable Message

Question

I am running Exchange 2010 on SBS 2011. Today, after trying to send an email outside my organization, I received the following Undeliverable reply from THEIR server, not mine (please note that, in the To field is the recipient's address, and in the CC field are two(2) emails that are within my domain):

Delivery has failed to these recipients or groups: CCemail1@localhost The e-mail address you entered couldn't be found. Please check the recipient's e-mail address and try to resend the message. If the problem continues, please contact your helpdesk. CCemail2@localhost The e-mail address you entered couldn't be found. Please check the recipient's e-mail address and try to resend the message. If the problem continues, please contact your helpdesk.

Diagnostic information for administrators:
Generating server: omikron.baseinvest.local
CCemail1@localhost
#< #5.1.1 550 5.1.1 <CCemail1@localhost>: Recipient address rejected: User unknown in local recipient table> #SMTP#
CCemail2@localhost
#< #5.1.1 550 5.1.1 <CCemail2@localhost>: Recipient address rejected: User unknown in local recipient table> #SMTP#
Original message headers:
X-Yandex-FolderName: xxxxxx
Received: from mxfront4h.mail.yandex.net ([127.0.0.1])
    by mxfront4h.mail.yandex.net with LMTP id olhFd8C4
    for <[e-mail]>; Mon, 29 Dec 2014 10:36:12 +0300
Received: from mail.mydomain.com (mail.mydomain.com [xxx.xxx.xxx.xxx])
    by mxfront4h.mail.yandex.net (nwsmtp/Yandex) with ESMTPS id kpw9RHS5if-aA2KqbKN;
    Mon, 29 Dec 2014 10:36:11 +0300
    (using TLSv1 with cipher AES128-SHA (128/128 bits))
    (Client certificate not present)
X-Yandex-Front: mxfront4h.mail.yandex.net
X-Yandex-TimeMark: 1419838571
Authentication-Results: mxfront4h.mail.yandex.net; spf=pass (mxfront4h.mail.yandex.net: domain of mydomain.com designates xxx.xxx.xxx.xxx as permitted sender) smtp.mail=[e-mail]
X-Yandex-Spam: 1
Received: from server.mydomain.local ([xxxx::xxxx:xxxx:xxxx:xxxx]) by
 server.mydomain.local ([xxxx::xxxx:xxxx:xxxx:xxxx]) with mapi id
 14.01.0438.000; Mon, 29 Dec 2014 09:36:06 +0200
From: ME <[e-mail]>
To: "[e-mail]" <[e-mail]>
CC: CCemail1 <[e-mail]>, CCemail2 <[e-mail]>
Subject: Issue regarding emails
Thread-Topic: Issue regarding emails
Thread-Index: AdAjODMvuvNcgfXlRhan5HKdNv1+6Q==
Date: Mon, 29 Dec 2014 07:36:05 +0000
Message-ID: <[e-mail]>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach: yes
X-MS-TNEF-Correlator:
x-originating-ip: [192.168.20.115]
Content-Type: text/plain
MIME-Version: 1.0
X-Yandex-Forward: 7433a6c2f7d3f171a74bcfe3725e87af

The thing that consider as strange is that this is a reply from their server, not mine, it tried to (probably) send something to the emails I had as cc but changed the domain to @localhost. Did anyone come across this kind of thing before? I sure did not!

Answer 1

I don't see what is strange about this – you attempted to deliver email to their system and it rejected the email because the recipients did not exist.

It’s possible (probable) that their software is rewriting the mailbox (as indicated by the @localhost) and getting confused – I do note that CC is visible in the headers for all recipients (unlike BCC) – so probably their system was incorrectly trying to deliver mail for these recipients as local. (It also means your mail server is doing nothing wrong, and its specs/details are irrelevant).