Manage a mailbox when a Microsoft 365 mailbox exists but an on-premises mail-enabled user doesn't exist

Original KB number:   2813304

Introduction

This article describes how to manage a mailbox in the on-premises environment in the following scenario:

• A Microsoft 365 mailbox exists.
• Active Directory synchronization is enabled.
• An on-premises mail-enabled user doesn't exist for the mailbox.

Procedure

To manage a mailbox in the on-premises environment in the scenario that's described in the "Introduction" section, follow these steps:

1. Use SMTP matching to match the Microsoft 365 user account with an on-premises user account. To do this, see How to use SMTP matching to match on-premises user accounts to Microsoft 365 user accounts for directory synchronization.

2. Convert the on-premises user account to a mail-enabled user. To do this, use Exchange Management Shell on the on-premises Microsoft Exchange server to run the following command:

   Enable-MailUser -Identity User -ExternalEmailAddress <user>@<domain>.mail.onmicrosoft.com
   

3. Obtain the Exchange GUID of the Microsoft 365 mailbox. To do this, follow these steps:

   1. Connect to Microsoft Exchange Online by using remote PowerShell. For more information, go to Connect to Exchange Online PowerShell.

   2. Run the following command:

      Get-Mailbox -Identity <UserAlias>
      

4. Update the on-premises mail-enabled user to use the Exchange GUID of the Microsoft 365 mailbox. To do this, use Exchange Management Shell to run the following command:

   Set-MailUser -Identity <UserAlias> -ExchangeGuid <Office365MailboxGUID>
   

5. Move the mailbox from Microsoft 365 to the on-premises domain. You can do this by using the Exchange Management Console or the Exchange Management Shell.

   For example, use the Exchange Management Shell to run the following command:

   New-MoveRequest -Identity <UserAlias> -OutBound -RemoteTargetDatabase <OnPremisesDatabase> -RemoteHostName <mail.domain.com> -RemoteCredential (get-credential) -TargetDeliveryDomain <domain.com>
   

   At the credential prompt, type the user name and password of the on-premises administrator to start the migration.

More information

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