Exchange 2003 - 550 5.7.1 Unable to relay

Hello Experts!,  I have a scenario that I need some help with:
#5.7.1 smtp;550 5.7.1 Unable to relay
We have an exchange server 2003 sp2.   We have two email addresses for our users and  For the most part this has been working fine for the past couple of years.
Recently, we've been getting several external customers telling us that when they email us at the .net email address, they get a bounce back message, but they're able to send it to the .com email address with no problems.    (we've only gotten this complaint from a handful of external customers from various companies).  Here's what confuses me,  it's not all or none---most customers can send to the .net and the .com,....only a select few external customers.  at first, it thought it might be on the customers end, but now we're seeing several customers with the same issue---its looking more like its on our end. Any clues?
I've copied a bounce back message from one of the external customers:
Diagnostic information for administrators:
Generating server:
#< #5.7.1 smtp;550 5.7.1 Unable to relay> #SMTP#
Original message headers:
Received: from ([]) by
 with Microsoft SMTPSVC(6.0.3790.3959);   Mon, 8 Mar 2010 15:08:44 -0800
Received: from
 ([fe80:0000:0000:0000:2df2:c6b4:]) by
 ([]) with mapi; Mon, 8 Mar 2010 15:08:39 -0800
From: Tim <>
To: 'Chris' <>
Date: Mon, 8 Mar 2010 15:08:39 -0800
Subject: Checking in . . .
Thread-Topic: Checking in . . .
Thread-Index: Acq/FEnfrA64GUU0Sp21tkPYkhBRNQ==
Message-ID: <>
Accept-Language: en-US
Content-Language: en-US
acceptlanguage: en-US
Content-Type: multipart/alternative;
MIME-Version: 1.0
X-OriginalArrivalTime: 08 Mar 2010 23:08:44.0251 (UTC) FILETIME=[4CD132B0:01CABF14]
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seven45Author Commented:
The unable to relay error would imply someone was trying to either send an email out through your mail server from another mail server or that the domain name on the outgoing email did not match the email servers domain. Do you have a reverse lookup for both domain names pointing back to your mail server?
Try for quick lookups.
Alan HardistyCo-OwnerCommented:
You are welcome.
Can you please describe your environment a little more.
  • What sort of firewall do you have?  Anything Cisco involved?
  • Any Anti-Spam / Anti-Virus software on the server?
  • Is the problem restricted to the same sending domain (all users from this domain) or random domains / random users?
Making Bulk Changes to Active Directory

Watch this video to see how easy it is to make mass changes to Active Directory from an external text file without using complicated scripts.

make sure that your recepient policy in the has (if yes)
make sure the user has teh email address stamped for which we are getting the NDR (if yes)
using adsiedit tool
check  (LM -- > SMtpSvc -- > 1 -- > Domain)
make sure that is listed in there (if yes)
force replication between DC
Marius GunnerudSenior Systems EngineerCommented:
I had a similar issue with one of my clients. are the emails running off of the same server or do you have different servers for each domain?

there were two things i did to sort this issue (given that we only had one domain).

I had my ISP setup a reverse PTR for the domain.

When you telnet to 25 (or .com) what is stated at the top. It should say something like 220 "" Microsoft Mail Service.....etc.

Now if it says something like 220 servername.domain.local.... then there is a problem as when remote servers try to contact you the server responds with the internal server name and not the public one. If this is the case here is what i did to resolve the issue:

For Exchange 2003 (not 100% sure but think this is where it is located)
1. Open System Manager
2. Expand Administrative Groups > First Administrative Group > Servers > server name > Protocols > SMTP
3. Right click Default SMTP Virtual Server
4. Select the Delivery Tab
5. Click Advanced
6. Make sure the correct FQDN is entered

For Exchange 2007
1. Open Exchange 2007 Console
2. Expand Server Configuration
3. Select Hub Transport
4. On the Receive Connectors Tab right click Default and choose properties
Change the FQDN on the General tab to the external name
5. Select Authentication tab
6. Unselect Exchange Server Authentication
7. Click Apply

Another possibility is that you need an SPF record configured as more and more servers are starting to require them. Take a look at this link to set this up: 


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In the NDR does it mention a server name next to the 5.7.1? Is it ? is that your Exchange 2003 server?

Do you only have one E2k3 server that hosts both domains?
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