550 5.1.1 User unknow RRS feed

  • Question

  • Dears,

    I am running exchange 2010 for around 2,800 mailboxes with no issues, however since 3 weeks external senders started getting "550 5.1.1 User unknown" for our newly created mailboxes , all the other addresses are perfectly fine. I did all the checks and traced everything is 100% normal. Can anyone help me on this matter please.

    Thursday, July 6, 2017 5:45 AM

All replies

  • Hi,

    It’s recommended to check if any different properties between the old mailbox and new created mailboxes, especially the Legacy DN.

    Please make sure the AD has synced.

    Please check if the external user received NDR message the same as below:


    Diagnostic information for administrators:

    Generating server: server.domain.com

    IMCEAEX-_O=FIRST+20ORGANIZATION_OU=EXCHANGE+20ADMINISTRATIVE+20GROUP+20+28FYDIBOHF23SPDLT+29_CN=RECIPIENTS_CN=James+20McClements23c@mydomain.com  #550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##


    If yes, please refer to my case for the steps to add the X500 address: https://social.technet.microsoft.com/Forums/office/en-US/2001bebb-b65e-4a97-b5a1-be8f5f8ac35b/local-email-exchange-2007?forum=exchangesvrsecuremessaging

    Check if this issue only affect specific external domain or all of the external domains.

    And check if the internal user can send messages to the newly created mailboxes.

    If it still doesn’t work, please post out the detailed NDR messages in detail.

    Hope it helps.


    Jason Chao

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    Friday, July 7, 2017 3:21 AM