Remote Mailboxes - Hybrid Config - Missing

The Remote Mailbox exists on the On Prem Exchange server and linked to the Office 365 mailbox. Without one of these for each Office 365 mailbox, you won't be able to configure Outlook to connect to the mailbox. The most common scenario that causes this issue is when you create a new user simply by creating the AD account, synchronizing it using DirSync/AADSync, and then licensing the user without creating a mailbox on prem and migrating it. This will give you a mailbox in Office 365, but does not create the remote mailbox on prem.

The correct way to provision new users in Office 365 is to create new Remote Mailboxes.

If a Remote Mailbox isn’t present or has been accidentally deleted, you can create one and link it up to the Office 365 mailbox.

We can resolve this manually by completing the following steps:

From the Exchange Management Powershell (On-Prem)

Enable-RemoteMailbox username –RemoteRoutingAddress alias@domain.mail.onmicrosoft.com

Get the Mailbox GUID of the Office 365 mailbox (O365 Powershell)

Get-Mailbox –Identity emailaddress | fl Identity,ExchangeGUID

Copy the Mailbox GUID and place this into Exchange Management Sell (On-Prem)

Set-RemoteMailbox username –ExchangeGUID "GUID Copied from Last Step"

After you have completed these steps you should force an AD synchronization from DirSync