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:
- Open Active Directory Users & Computers. Locate the user account and clear the msExchHomeServerName attribute.
- Open Exchange Management Shell on an on-premises Exchange Server, then type the command:
Enable-MailUser -Identity email@example.com -ExternalEmailAddress firstname.lastname@example.org
Where email@example.com is the email address of the user account and firstname.lastname@example.org 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.
- Continue in Exchange Management Shell with:
Enable-RemoteMailbox -Identity email@example.com
- Open Exchange Online PowerShell and connect to your tenancy, then type the command:
Get-Mailbox -Identity firstname.lastname@example.org | Format-Table -Property Identity, ExchaneGuidNote down the ExchangeGuid that is returned.
- Back in Exchange Management Shell, type:
Set-RemoteMailbox -Identity email@example.com -ExchangeGuid "The ExchangeGuid from step 4"
- 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.