SMTP via User


Our CFO has Multi-Factor authentication set up and working. Someone tried to send an email on his behalf to a non-existent email address, so he received the bounce back in his inbox. The original email was never in his Sent box, but how is it possible that someone (a hacker I presume) could send email on his behalf, through Office365, without authenticating as him?


Here are the message headers, I replaced the sender (our CFO) with YYYYY and the recipient (a non-existent mailbox) with XXXXX

Original Message Headers
Received: from
 (2a01:111:e400:51e4::28) by
 (2a01:111:e400:530f::21) with Microsoft SMTP Server (version=TLS1_2,
 cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384_P256) id 15.20.548.13; Wed, 7
 Mar 2018 15:43:47 +0000
Received: from
 (2a01:111:f400:7e51::208) by
 (2a01:111:e400:51e4::28) with Microsoft SMTP Server (version=TLS1_2,
 cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.567.14 via Frontend
 Transport; Wed, 7 Mar 2018 15:43:47 +0000
Authentication-Results: spf=neutral (sender IP is;; dkim=pass
 (signature was verified);;
 dmarc=none action=none;
Received-SPF: Neutral ( is neither
 permitted nor denied by domain of
Received: from ( by ( with Microsoft SMTP
 Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id
 15.20.548.7 via Frontend Transport; Wed, 7 Mar 2018 15:43:46 +0000
Received: from compute7.internal (compute7.nyi.internal [])
        by mailnew.nyi.internal (Postfix) with ESMTP id 573EE10F1
        for <>; Wed,  7 Mar 2018 10:43:46 -0500 (EST)
Received: from frontend1 ([])
  by compute7.internal (MEProxy); Wed, 07 Mar 2018 10:43:46 -0500
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=; h=content-transfer-encoding:content-type
        :x-me-sender:x-me-sender:x-sasl-enc; s=fm2; bh=J5yc94p8OcoMielqo
        weJHr1/JS5dWOFLsW5ZI0n+giI=; b=Sros9ppkL1hz/XZGS/A7gcjWZy4Q1fdOB
X-ME-Sender: <xms:sgigWt3l2Il5qJP0vi8x-g3P3mmsCsFjIkz3MPBYz2AwZZnY_PnjZA>
Received: from Ms-MacBook.local (unknown [])
        by (Postfix) with ESMTPA id 9D2167E660
        for <>; Wed,  7 Mar 2018 10:43:45 -0500 (EST)
From: Chris YYYYY <>
Subject: Kindly get back
Message-ID: <>
Date: Wed, 7 Mar 2018 10:43:44 -0500
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:52.0)
 Gecko/20100101 Thunderbird/52.6.0
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Content-Language: en-US
X-EOPAttributedMessage: 0
X-EOPTenantAttributedMessage: 9c9ca00d-d89b-441e-a989-1ae7f6387804:0
X-Forefront-Antispam-Report: CIP:;IPV:NLI;CTRY:US;EFV:NLI;
X-Microsoft-Exchange-Diagnostics: 1;DM3NAM05FT022;1:22KTfsIJsrHtk/9hXI7qkXbLBstTxAbLpPufCoqKHWGJ4egiyN9wLIV2Evy2BOE4ZyF7IR3XeGyx94qwyUTSJ4yzyLM4x1stQhhuaziR6t+CkNW6DCOra22VCBBlc7/L
X-MS-PublicTrafficType: Email
X-MS-Office365-Filtering-Correlation-Id: c30401f0-1aeb-4fd8-d598-08d58442372f
X-MS-TrafficTypeDiagnostic: SN1PR11MB0574:


6 Replies

Looks like a typical spoof attempt to me. Anyone can send anything as anyone on the internet when it comes to SMTP. I could pop out to my SMTP server and send an e-mail as if I wanted to to anyone I wanted. If they aren't using DKIM or SPF etc. it could very well get through, but in this case it was blocked and returned whom the message was set as the from address.


This is hard for me to believe, simply because I have devices on my network that I have set up to send alerts via SMTP, and if I do not authenticate as the same email as the sender it fails to send the email. SMTP via O365 seems to be very picky on what it accepts, simply because SMTP use to be so open and allow for anyone to send anything. I thought that was fixed now.

This email originated from some other IP outside of 365. You don’t have to use their servers to send as their domains.

So you are saying that their servers allowed them to send email from our O365 domain to us, using their SMTP servers, not ours? If so, wouldn't it show the email address they used to authenticate against their SMTP server, or they just sent it anonymously with no authentication? 

They didn’t send anything to your servers you said it got blocked and then you got the NDR. I can go right now to my smtp server and send as you to some random place and if it got blocked you would get the NDR.

oh, we got a bounce back because the TO email address does not exist, not because it was blocked as spam. The email address they were sending to was not valid. If it would have been, the email would have been delivered and we would have never known about it.