550 5.1.10 Bounce from user whom the email was not sent to.

Przemyslaw Wawrzyczny 21 Reputation points
2022-11-14T10:02:25.807+00:00

Hello Guys,

I wonder if any of you have experienced a similar problem and will be able to advise on how to fix it.

It is an intermittent issue.

Some users report that they receive following Bounce message:

Generating server: Exchange Serer
userX@keyman .ac.uk
Remote Server returned '550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup'

The bounce is regarding user to whom the message was not sent to.
The user have been removed from AD few months ago.

How it is possible that when user sends email it sometimes goes to userX as well?

Regards,
Przemek

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3 answers

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  1. Amit Singh 4,846 Reputation points
    2022-11-15T11:30:08.267+00:00

    This problem occurs if the user was created in Microsoft 365. In this scenario, the on-premises Exchange environment has no object to reference for the user. Therefore, all queries for that SMTP address fail.

    You can check these articles for more insight -
    https://learn.microsoft.com/en-us/exchange/troubleshoot/email-delivery/recipientnotfound-ndr
    https://www.stellarinfo.com/blog/fix-email-delivery-ndr-issues-error-550-exchange-server/

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  2. Przemyslaw Wawrzyczny 21 Reputation points
    2022-11-15T11:58:47.78+00:00

    Hi AholicLiang-MSFT,

    I appreciate your help in this case.

    We have Exchange Hybrid (Exchange Online + Exchange 2016)

    I most cases user replays to the email communication, so each email Title starts with RE: , userX is not selected as recipient.

    I have carefully checked all enabled mail flow rules (on both exchange online and exchange 2016) and did not find anything regarding userX.

    When doing a trace in exchange Online for each reported case, I can see that email was also sent to userX even though userX was not recipent.
    I have few copies of these emails and userX is not there.

    When checking these emails on our 3th party Email Security Platform I can see these emails are coming Inbound through Email Security Platform to Exchange 2016, the NDR message says Exchange 2016. (usually if the email is sent to corporate user that communication is not visible on 3th pary Email Security Platform, only incoming and outgoing emails which traverses through Email Security platform).

    The interesting fact is that not everyone experience this issue (from the logs I know that around 25 ppl has received that NDR message so far)

    Could this be caused by misconfiguration somewhere?

    Regards,
    Przemek

  3. Aholic Liang-MSFT 13,741 Reputation points Microsoft Vendor
    2022-12-08T08:00:09.567+00:00

    Hi @Przemyslaw Wawrzyczny ,

    Apologise for late reply, the issue has been resolved.

    Great to know that you've already resolve your issue and really appreciate it for your sharing, so that others experiencing the same thing can easily reference this!
    By the way, since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others.". and according to the scenario introduced here: Answering your own questions on Microsoft Q&A, I would make a brief summary of this thread:
    * [550 5.1.10 Bounce from user whom the email was not sent to.] **

    **Issue Symptom: **
    It is an intermittent issue.
    Some users report that they receive following Bounce message:
    Generating server: Exchange Serer
    userX@keyman .ac.uk
    Remote Server returned '550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup'
    The bounce is regarding user to whom the message was not sent to.
    The user have been removed from AD few months ago.
    How it is possible that when user sends email it sometimes goes to userX as well?

    The Solution:

    I spotted that all of the messages he was sent had a spam flag within the data when you looked on the Exchange trace. I also noticed that the messages to userX were a forward of the original message, which led me to think there must be a rule somewhere other than Exchange online (which I'd already checked). After a bit of googling I was pointed to the Security area of Azure where there are anti-spam policies. In the outbound anti spam policy I found the forward to userX.

    You could click the "Accept Answer" button for this summary to close this thread, and this can make it easier for other community member's to see the useful information when reading this thread. Thanks!

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