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Exchange 2013 : 550 5.1.1 User unknown for many recipients except just one recipient address is wrong

    Question

  • Hello,

    We have a strange error with our Exchange 2013 Server (CU13).

    When an external user sends mail to 2 recipients which have same domain (samedomain.fr for example), if an address is wrong, the sender receives an error (see below) says 2 address are unknows. 

                       The mail system

    <test3@samedomain.fr>: host mail.samedomain.fr[X.X.X.X] said: 550 5.1.1 User
        unknown (in reply to end of DATA command)

    <toto@samedomain.fr>: host mail.samedomain.fr[X.X.X.X] said: 550 5.1.1 User
        unknown (in reply to end of DATA command)

    Any ideas ?

    Thanks for help.

    Benoît

    Tuesday, April 25, 2017 1:31 PM

All replies

  • Make sure that your Exchange server is the one doing this.  You might get some clues by turning up protocol logging on the appropriate receive connector and examining the logs at the time of a message bounce.  Or you can telnet to the server's port 25 and compose a message by hand that matches this scenario to see what is actually happening.

    Ed Crowley MVP "There are seldom good technological solutions to behavioral problems."
    Celebrating 20 years of providing Exchange peer support!

    Thursday, April 27, 2017 6:49 AM
    Moderator
  • Hi Benoît,

    Beside the suggestion above, I would ask the sender's exchange admin to check if there is any transport rule in his domain, which would cc/bcc the message to another wrong address. 

    Best Regards,


    Niko Cheng
    TechNet Community Support


    Please remember to mark the replies as answers.
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    Thursday, April 27, 2017 12:28 PM
    Moderator
  • I'd say it's more likely that the other server is generating a misleading non-delivery report.

    Ed Crowley MVP "There are seldom good technological solutions to behavioral problems."
    Celebrating 20 years of providing Exchange peer support!


    Thursday, April 27, 2017 11:08 PM
    Moderator
  • We've the same issue here. I don't know if it's helpful, but I assume that this is kinda spam protection on the receiving Exchange server. If a mail is sent to different recipients within the same domain, and at least one recipient address is incorrect, the mail server rejects the mail to all of the recipients. As soon as the incorrect addres(ses) is/are removed, it works fine.

    This behavior is probably controlled by RecipientValidationEnabled switch within RecipientFilterConfig but I did not dig in any further, because from my pov it's a nice anti spam feature.

    Holger


    • Edited by Holger Keil Monday, February 12, 2018 9:56 AM
    • Proposed as answer by Mihail Borisov Thursday, April 11, 2019 10:32 PM
    Monday, February 12, 2018 9:55 AM
  • Again, you should be able to verify this with message tracking and the SMTP protocol log.

    Ed Crowley MVP "There are seldom good technological solutions to behavioral problems."
    Celebrating 20 years of providing Exchange peer support!

    Tuesday, February 13, 2018 2:25 AM
    Moderator
  • That pinpointed the right answer. I had exactly the same issue that is resolved now. In order to disable this although not recommended as this is a nice to have anti-spam feature:

    1. In Exchange Management Shell run:

        Set-RecipientFilterConfig -RecipientValidationEnabled $false

    2. Restart the transport service (Microsoft Exchange Frontend Transport) under Services.

    3. Ensure RecipientValidationEnabled is disabled:

        Get-RecipientFilterConfig | FL 

    Mihail

    Thursday, April 11, 2019 10:32 PM