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No User Account Equals No Access to Group Mailboxes
A somewhat disgruntled reader of the Automating Microsoft 365 with PowerShell eBook emailed me to ask why the Microsoft Graph Outlook Mail API doesn’t support group mailboxes. For example, you can’t do this:
$UserId = Get-UnifiedGroup -Identity "Ultimate Guide to Microsoft 365" | Select-Object -ExpandProperty ExternalDirectoryObjectId Get-MgUserMailFolder -MailFolderId Inbox -UserId $UserId Get-MgUserMailFolder_Get: The requested user '33b07753-efc6-47f5-90b5-13bef01e25a6' is invalid. Status: 404 (NotFound) ErrorCode: ErrorInvalidUser
The commands are straightforward. The Get-UnifiedGroup cmdlet fetches the Entra ID object identifier for the Microsoft 365 group associated with the group mailbox and the Get-MgUserMailFolder cmdlet attempts to find the inbox folder in the mailbox. The Graph responds that the requested user is invalid.
My reasoned explanation didn’t make my questioner happy. A group mailbox is attached to an Entra ID group object, not a user account. As the documentation says “The Microsoft Graph API supports accessing data in users’ primary mailboxes and in shared mailboxes.” Without a user account linked to the mailbox, the Graph rejected the request.
The Graph APIs currently support access only to mailboxes attached to user accounts. Shared mailboxes have disabled user accounts and can be accessed in the same way as regular user mailboxes. Other types of mailboxes used by Exchange Online, such as the special online mailboxes used to hold compliance records for guest accounts, are also inaccessible.
Archive Mailbox Access Coming
The same page mentions that the API doesn’t currently support access to archive mailboxes. This situation is due to change as part of Microsoft’s long-running project to remove Exchange Web Services from Exchange Online. However, Microsoft hasn’t yet given a date for the Graph API to support archive mailboxes.
In passing, MC1446796 (3 August 2026) highlights that the final deprecation process for Exchange Web Services swings into operation from October 1, 2026, when a bunch of hybrid organization features that currently depend on EWS are gradually switched off in a rolling update across Exchange Online.
Client Access to Group Mailboxes
Outlook clients can access the contents of group mailboxes, but they don’t use the public Graph APIs to do so. Instead, Outlook relies on a mixture of internal APIs and MAPI, depending on the client.
This led to a further question about why Microsoft publishes a Mail API that cannot interact with group mailboxes like clients do. Even going back to the earliest days of Outlook and the MAPI API, the software used a mixture of public and undocumented APIs. The reality is that Microsoft clients often rely on internal APIs to deliver functionality that isn’t available through publicly documented interfaces.
The same is true of Exchange PowerShell where internal APIs power cmdlets like Get-MailboxFolderStatistics that can run against group mailboxes:
Get-MailboxFolderStatistics -Identity '33b07753-efc6-47f5-90b5-13bef01e25a6' | Format-Table Name, ItemsInFolder Name ItemsInFolder ---- ------------- Top of Information Store 2 Archive 0 Calendar 10 Deleted Items 23 Drafts 0 Inbox 8886 Junk Email 24 MeContact 0 Sent Items 493
Can’t Build Outlook from the Outlook Mail API
The Outlook Mail API is very good at tasks like retrieving folders, messages, calendar items, and tasks. However, a client built exclusively on the public API would be a poor substitute for any Outlook client.
Given that Outlook classic has been under active development since 1996, it’s unsurprising that the client is so feature-rich, which is one of the reasons why Microsoft is struggling to convince users to move to the New Outlook client.
Maybe Not Such a Big Deal
Returning to group mailboxes, the percentage of customers who use them for email-based discussions, as originally envisaged for Outlook Groups in 2014, is probably quite small today. I still have a few email-based discussion groups that I access through Outlook (Figure 1), but most of the workload has moved elsewhere.
The most important role for group mailboxes is to support Microsoft 365 workloads such as Teams through calendar data and the storage of compliance records for channel conversations. With that in mind, perhaps the lack of Graph API access to the folders and items stored in group mailboxes is probably less important than it seems.
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