Link a Synced Active Directory User to an Existing Microsoft 365 Mailbox in a Hybrid Environment

In this scenario, an on-premise AD user was created and synced with Microsoft 365. The Microsoft 365 user has an Exchange Online mailbox, but this mailbox is not visible in the on-premise Exchange Admin Center.

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Instructions

• Obtain the GUID of the Exchange Online Mailbox
• Update the On-Premise Exchange Environment

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Obtain the GUID of the Exchange Online Mailbox

1. Open PowerShell and connect to Exchange Online.

2. Use the Get-Mailbox command to obtain the GUID of the user's existing mailbox in Exchange Online. Save the GUID for use in the next section.

   Get-Mailbox jdoe | Format-List ExchangeGuid
   
   ExchangeGuid : < ExchangeGuid >
   

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Update the On-Premise Exchange Environment

1. Open Exchange Management Shell on the on-premise Exchange server.

2. Use the Enable-RemoteMailbox command to mail-enable the on-premise user. The -RemoteRoutingAddress parameter value will be the user's @<tenant>.mail.onmicrosoft.com SMTP address.

   Enable-RemoteMailbox jdoe -RemoteRoutingAddress jdoe@acme.mail.onmicrosoft.com
   
   Name        RecipientTypeDetails    RemoteRecipientType
   ----        --------------------    -------------------
   John Doe    RemoteUserMailbox       ProvisionMailbox
   

3. Use the Set-RemoteMailbox command to update the GUID of the on-premise mailbox to match the GUID of the mailbox in Exchange Online. Use the value obtained in Step 2 of the last section.

   Set-RemoteMailbox jdoe -ExchangeGuid <ExchangeGuid>