Office 365 Hybrid – On Premise Room Mailboxes not available in OWA

I came across an issue today wherein a user whose mailbox was hosted on Office 365 attempted to use OWA to book a meeting On Premise, and was told that there were no rooms available. The list simply didn’t contain any rooms. The Room Mailboxes were synchronised to Office 365 using the AADSync tool, however OWA knew nothing about them. When using Outlook, the Rooms were shown correctly, so this was just an issue with OWA, not with Room Mailboxes per se.

After a little digging, I found the following KB article: which explains that a Room List should be created and synchronised to Office 365 to get this working in OWA.

However the PowerShell cmdlet in the KB article fell a little short as I had over 100 Rooms to add into the Distribution Group. This was the PowerShell I ended up using:

$members=Get-Mailbox -RecipientTypeDetails RoomMailbox
New-DistributionGroup -Name "RoomList" -RoomList -Members $Members

I then moved the newly created Distribution Group into the correct OU and performed a Directory Sync. The RoomList showed up instantly, but it took 5-10 minutes for it to become populated. Once this was all done, the Rooms were available!



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 🙂