MigratedMessages folder for Office 365 Group mailboxes

Here’s something interesting I spotted a while back – all Office 365 Group mailboxes in my tenant seem to have an additional (sub)folder under the RecoverableItems subtree, named “MigratedMessages”. And only Office 365 Group mailboxes, none of the other types seem to have it. You can run the following cmdlet to check the presence of this folder for your Group mailboxes:

Get-UnifiedGroup | % { Get-MailboxFolderStatistics $_.PrimarySmtpAddress -FolderScope RecoverableItems | ? {$_.Name -eq "MigratedMessages"} | select Name,Identity,FolderSize }

Name             Identity                                           FolderSize
----             --------                                           ----------
MigratedMessages Unified@michev.info\MigratedMessages               0 B (0 bytes)
MigratedMessages firstgroup@michev.onmicrosoft.com\MigratedMessages 0 B (0 bytes)
MigratedMessages chinese@michev.info\MigratedMessages               0 B (0 bytes)
...

In my case, every single Group mailbox has this folder, and in every single Group mailbox this folder is empty. Zero messages, zero size. Zero results online as to what it might be used for. And the best part – the folder has been around since at 2014 at the very least, according to entries from my PowerShell transcripts. My initial guess, based on the name of the folder and the fact that it has existed for years is that it might have something to do with the initial release of the Unified Groups feature, and maybe even some plans Microsoft had to offer (shared) mailbox “conversion” into Groups. Which doesn’t explain why this folder is present for newly created Groups as well.

While looking at this folder, I discovered yet another new entry under the RecoverableItems subtree. Namely, a folder named “SubstrateHolds” seems to be present in some of the Group mailboxes. Some, as in not all of them have it. No “regular” mailbox has it either. Here’s how to check for it:

Get-UnifiedGroup | % { Get-MailboxFolderStatistics $_.PrimarySmtpAddress -FolderScope RecoverableItems | ? {$_.Name -eq "SubstrateHolds"} | select Name,Identity,FolderSize }

Name           Identity                                         FolderSize
----           --------                                         ----------
SubstrateHolds firstgroup@michev.onmicrosoft.com\SubstrateHolds 0 B (0 bytes)
SubstrateHolds chinese@michev.info\SubstrateHolds               0 B (0 bytes)
SubstrateHolds OutlookGroup@michev.info\SubstrateHolds          0 B (0 bytes)

As with the “MigratedMessages” folder, there is absolutely no information available what the folder is used for. Its name and the fact that it’s located under the RecoverableItems subtree suggests that it has something to do with the hold functionality, but what exactly is beyond me. If I ever manage to get additional info, I’ll make sure to update the post 🙂

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