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RoyalDCosta-9872 asked LucasLiu-MSFT commented

Mail NDR errors in Hybrid Exchange setup

Hello All

i am on project for migrating 2000 mailboxes to Exchange online, i have two Exchange forest environments on premises running Exchange 2013 and 2016 respectively, i am facing a unique problem of NDR these days . scenario is as follows.

For new joiners i am creating user account in AD(on Premises) and allow it to sync and then create Mailbox on cloud.

Challenge - existing On premises users are getting NDRs for mails sent to newly created mailboxes on cloud.

can someone help me to resolve this.

office-exchange-server-mailflow
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What is the exact NDR?

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Following is the NDR

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image.png (11.9 KiB)

Hi @RoyalDCosta-9872 ,
I am writing here to confirm with you how thing going now?




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Hi @RoyalDCosta-9872 ,
I am writing here to confirm with you how thing going now?




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joyceshen-MSFT avatar image
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joyceshen-MSFT answered

Hi @RoyalDCosta-9872

Agree with the reply above, please make sure you have followed the correct way to create a new o365 mailbox in Exchange hybrid. Several ways available list in the below article: Creating an Office 365 Mailbox in a Hybrid Configuration

Here is also a related thread discussed the similar issue as yours: There's a problem with the recipient's mailbox. Please try resending the message. If the problem continues, please contact your email admin.

For this issue we need to check if there are any Internet recipient entry in the domain.
There was an AD account for which email address was added by mistake. Exchange was unable to deliver and was throwing the error. We remove the email address off the AD account and it got resolved..

In addition, if you mistakenly have duplicated mailboxes both in Exchange on-prem and online, we will need to follow the guide here to resolve this issue: My user has a mailbox both on-premises and in Exchange Online. Help!


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AlexC-8264 avatar image
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AlexC-8264 answered AlexC-8264 commented

You probably enabled the mailbox in the cloud only, right?
Did you run: Enable-RemoteMailbox "Test User" -RemoteRoutingAddress "test.user@mydomain.mail.onmicrosoft.com" ?
If not, your on-prem Exchange doesn't know where to route the e-mail.

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Thanks for the reply. for which user shall i run this command? sender or target?

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for whom you want a mailbox to be prepared in the o365 / Exchange online.

First, disable the cloud only mailbox again.
then, run enable-remotemailbox on-prem.

Refenrence: https://docs.microsoft.com/en-us/powershell/module/exchange/enable-remotemailbox?view=exchange-ps

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