Mail NDR errors in Hybrid Exchange setup

Royal D Costa 241 Reputation points

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.

Exchange Server Management
Exchange Server Management
Exchange Server: A family of Microsoft client/server messaging and collaboration software.Management: The act or process of organizing, handling, directing or controlling something.
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  1. AlexC 246 Reputation points

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

  2. Joyce Shen - MSFT 16,631 Reputation points

    Hi @Royal D Costa

    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!

    If an Answer is helpful, please click "Accept Answer" and upvote it.

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