Remote Mailboxes in Exchange Hybrid configuration

I’ve been asked a few questions recently about Remote Mailboxes in Office 365 hybrid configurations. The Remote Mailbox exists on the On Premise Exchange server and is the link between the Office 365 mailbox and the On Prem Exchange Organisation. Without one of these for each Office 365 mailbox, you can’t effectively manage certain Office 365 mailbox properties, you can’t offboard it back to the On Prem Exchange Server, and most importantly, not having a Remote Mailbox breaks mail flow between users On Prem and users in Office 365.

Quite often, when administrators first start using Office 365 in Hybrid mode, they will create a new user simply by creating the AD account, synchronising it using DirSync/AADSync, and then licensing the user. This will give you a mailbox in Office 365, but will also cause the problems listed above. 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.

To do this:

From Exchange Management Shell (On Premise):

Enable-RemoteMailbox username –RemoteRoutingAddress

The RemoteRoutingAddress is always in the format of, for example:

Enable-RemoteMailbox joeb –RemoteRoutingAddress

You then need to get the Mailbox GUID of the Office 365 mailbox. To do this, go into Office 365 PowerShell and run:

Get-Mailbox –Identity emailaddress | fl Identity,ExchangeGUID

Copy the Mailbox GUID into your clipboard and go back to the Exchange Management Shell (On Premise):

Set-RemoteMailbox username –ExchangeGUID 8e992097-24c1-432c-8a89-98e3c7a7d283

Anything in italics needs to be changed to a parameter relevant to your requirements. Once you’ve completed this, perform a delta/incremental sync and the two shall become one (so to speak!)

There is a KB article from Microsoft on a similar issue (trying to Offboard a mailbox where the Remote Mailbox GUID is not the same as the 365 GUID) here:

Thanks for reading 🙂