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Email Rejected by one system with #5.0.0 smtp;555 error

Hello experts!

I have an email sending problem and I am hoping one of you will have some insight that can help me solve this problem

My company has a new client, but all emails from us to them are rejected with a 555 error. My research so far suggests this means a protocol mismatch, but I don't really know what that means.

This happens for emails sent to this customer from any mailbox in our organization. We do not have this problem when sending to any other people or companies and our system has been up for years.

Email sent from my user's personal email account and from my Gmail account is received successfully.

I tried creating a special send connector just for this domain that Forces Helo instead of Ehlo, but that did not fix anything.

Our email servers are all Exchange 2010 with Service Pack 2 and Update Rollup 6. Mail failures occur whether we send with Outlook 2010 client or OWA.

Bounce message follows:

Sent: Friday, August 30, 2013 9:10 AM
To: IT Admin
Subject: Undeliverable: RE: Test Email

Delivery has failed to these recipients or groups:

A problem occurred during the delivery of this message to this e-mail address. Try sending this message again. If the problem continues, please contact your helpdesk.

The following organization rejected your message: company.com.cn.

Diagnostic information for administrators:

Generating server: bigfish.com

company.com.cn #<company.com.cn #5.0.0 smtp;555 Unsupported option: AUTH=<>> #SMTP#

Original message headers:

Received: from mail112-tx2-R.bigfish.com ( by
 TX2EHSOBE004.bigfish.com ( with Microsoft SMTP Server id; Fri, 30 Aug 2013 16:09:50 +0000
Received: from mail112-tx2 (localhost [])      by
 mail112-tx2-R.bigfish.com (Postfix) with ESMTP id 5E1B73A0171      for
 <newcustomer@company.com.cn>; Fri, 30 Aug 2013 16:09:50 +0000 (UTC)
X-Forefront-Antispam-Report: CIP:ww.xx.yy.zz;KIP:(null);UIP:(null);IPV:NLI;H:mail.mycompany.com;RD:none;EFVD:NLI
X-SpamScore: -1
X-BigFish: VPS-1(z569dhz9371Ic85fhdbeeh55dIzz208ch1ee6h1fdah2073h1202h1e76h1d2ah1fc6hzz1d7338h1de098h1b1984h6d524h17326ah18c673h186068h1954cbh18602eh8275bh1bc7b9h8275dh1de097hz31h2a8h839h8e2h8e3hd25hf0ah1288h12a5h12bdh137ah1441h14ddh1504h1537h153bh15d0h162dh1631h1758h18e1h1946h19b5h1a24h1a82h1ad9h1b0ah1b2fh1bceh1fb3h1d0ch1d2eh1d3fh1dc1h1dfeh1dffh1e1dh1fe8h1ff5hbe9i1155h)
Received-SPF: softfail (mail112-tx2: transitioning domain of mycompany.com does not designate ww.xx.yy.zz as permitted sender) client-ip=ww.xx.yy.zz; envelope-from=IT.Admin@mycompany.com; helo=mail.mycompany.com ;ompany.com ;
Received: from mail112-tx2 (localhost.localdomain []) by mail112-tx2
 (MessageSwitch) id 1377878934621385_7997; Fri, 30 Aug 2013 16:08:54 +0000
Received: from TX2EHSMHS034.bigfish.com (unknown [])      by
 mail112-tx2.bigfish.com (Postfix) with ESMTP id 846A32E0053      for
 <newcustomer@company.com.cn>; Fri, 30 Aug 2013 16:08:54 +0000 (UTC)
Received: from mail.mycompany.com (ww.xx.yy.zz) by
 TX2EHSMHS034.bigfish.com ( with Microsoft SMTP Server (TLS) id; Fri, 30 Aug 2013 16:08:52 +0000
Received: from MailServer.Company.corp ([fe80::f515:b53:b437:cb07]) by
 MailServer.Company.corp ([fe80::f515:b53:b437:cb07%14]) with mapi id
 14.02.0342.003; Sat, 31 Aug 2013 00:08:50 +0800
From: IT Admin <IT.Admin@mycompany.com>
To: "newcustomer@company.com.cn" <newcustomer@company.com.cn>
Subject: RE: Test Email
Thread-Topic: Test Email
Thread-Index: Ac6lkX32hYETuTz+SWiivEWUmUqrQgACbAhw
Date: Fri, 30 Aug 2013 16:08:49 +0000
Message-ID: <9E1A6C20647B694EBA6C500F52376EBD1D8D77A0@MailServer.Company.corp>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach: yes
x-originating-ip: [xxx.xxx.x.xx]
Content-Type: multipart/related;
MIME-Version: 1.0
Return-Path: IT.Admin@mycompany.com
X-OriginatorOrg: mycompany.com
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Last Comment
Simon Butler (Sembee)

8/22/2022 - Mon
Simon Butler (Sembee)

bigfish.com is a domain used by Hosted Forefront, Microsoft's hosted antispam solution. Are you their customer or is the client?


I am their customer. We use them as a smart host.
Simon Butler (Sembee)

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Hi Simon,

Thank you for your attention to this. Please tell me if I am understanding you correctly.

I think you are saying that our SmartHost sends the mail out with an AUTH verb that is particularly problematic for Postfix type servers. So if the recipient's server is a Postfix type server it might make sense to try making a custom send connector that just sends mail out directly without going through the SmartHost and scope the Send connector to handle this domain. The hope here is that Exchange does not use an AUTH verb and would not have the same problems as the SmartHost. Is that what you are suggesting?

I started with Experts Exchange in 2004 and it's been a mainstay of my professional computing life since. It helped me launch a career as a programmer / Oracle data analyst
William Peck

Thanks for your excellent expert help Simon!

In addition to being accurate, you were also quick and explained things well.
Simon Butler (Sembee)

If you use a smart host, then the auth verb shouldn't be passed to the remote server and cause the problem. Postfix is quite common but this is not a common issue. Therefore it has to be something about how the recipient has configured their server.