Hybrid Exchange: Adding a user with Exchange Online mailbox to on-premises Exchange Server

In Hybrid Exchange scenarios, if an Active Directory user has been synced to Microsoft 365 and granted an Exchange Online mailbox outside of creating an Office 365 mailbox in the hybrid Exchange ECP, the user will not appear in Exchange on-premises servers.

To fix this:

1. Open Active Directory Users & Computers. Locate the user account and clear the msExchHomeServerName attribute.
   
   
   
   
2. Open Exchange Management Shell on an on-premises Exchange Server, then type the command:
   

   Enable-MailUser -Identity user@company.com -ExternalEmailAddress user@m365tenant.mail.onmicrosoft.com

   
   Where user@company.com.au is the email address of the user account and user@m365tenant.mail.onmicrosoft.com is the routing address you want the user account to have in the hybrid Exchange environment. The routing address will not exist in Exchange Online for this user, you are setting it here, and will be synced to the service in step 6.
   
   
3. Continue in Exchange Management Shell with:
   

   Enable-RemoteMailbox -Identity user@company.com

4. Open Exchange Online PowerShell and connect to your tenancy, then type the command:
   

   Get-Mailbox -Identity user@company.com | Format-Table -Property Identity, ExchaneGuid

   Note down the ExchangeGuid that is returned.
   
   
5. Back in Exchange Management Shell, type:
   

   Set-RemoteMailbox -Identity user@company.com -ExchangeGuid "The ExchangeGuid from step 4"

6. On your server with AD Connect installed, run a Delta Sync:
   

   Start-ADSyncSyncCycle -PolicyType Delta

The on-premises Exchange ECP will now display an Office 365 mailbox for the user. Users with local mailboxes will be able to send to this recipient.