Tuesday, January 18, 2011

Smtp; 5.1.0 - Unknown address error 550-'5.7.1 Command rejected' (

Issue : Sending mail to specific recipient is failing with  SMTP XXX code.

Solution:  fallow the steps below

Delivery has failed to these recipients or distribution lists:


An error occurred while trying to deliver this message to the recipient's e-mail address. Microsoft Exchange will not try to redeliver this message for you. Please try resending this message, or provide the following diagnostic text to your system administrator.

The following organization rejected your message: [].

Diagnostic information for administrators:

Generating server: smtp02.smtp25.gov


[] #<[] #5.0.0 smtp; 5.1.0 - Unknown address error 550-'5.7.1 Command rejected' (delivery attempts: 0)> #SMTP#

More Explanation

If you get a ticket from your helpdesk or you are the Tier 1 person and have no idea what to do, keep reading as I will talk about very basic how to troubleshoot such issues.

Now let’s quickly understand couple basic mail flow principles. When sending mail, the sending server attempts to try to locate the mail, in reality it is up to receiver weather to receive such e-mail or not. Furthermore the recipient mail server could be generating whatever error code for it wants for whatever reason it has.

Forget about mail servers and now think about a postman, who works at the neighborhood post office. He loads his truck with bunch of mails and start delivering them, if you do not have dedicated post box in front of your home he drops the envelope in there. It is up to you whether you pick them up or not in this example.

Same post man might have another packet for you this time he won’t drop into mailbox in front of your home, he knocks on your door and, tries to get signature from you. Let’s suppose you open the door, postman told you, sir I have package for you, please sign the computer before I give it to you.

Now it is up to you to tell the post man, I don’t care about this package, I won’t sign anything just take this back? ( this is perfectly possible ) in this case post man will take the package and write note on it most likely say something like rejected by blah blah , 550-'5.7.1

Steps to Understand what went wrong

  • Now to investigate such issues the easiest thing to do go here www.testexchangeconnectivity.com
  • In here you will perform Inbound SMTP test
  • Put the destination recipient mail address here


Now you can see the reason why your mail server could not deliver the message, in reality no mail will route to this address as the recipient mail servers does have errors indicated in below picture


So now how you are going to fix this?

You really cannot fix this as you are not generating the errors ( your mail server) nor you are the administrator of recipient mail servers. I guess you can locate the technical support people for the destination SMTP address space and open ticket or talk to someone , saying, hey we have been trying to sent you guys mail your mail server is keep rejecting it , can someone fix these problems or tell us when you will accept mail.


The point I wanted to make is to lead some of you to develop troubleshooting skill and  process and stick to it. Many of errors in such easily can be identified by using online tools, Telnet from command line into port 25 to destination mail server or any other way.

Oz Casey, Dedeal
MCSE 2003, M+, S+, MCDST
Security+, Project +, Server +
http://smtp25.blogspot.com/ (Blog
http://telnet25.wordpress.com/ (Blog)

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