Multiple mailbox users match identity “Mailbox1”

So I’m working on a nice Public Folder migration from Exchange 2016 to Exchange Online. Last week all preparations were performed and this morning I was planning to start the migrationbatch. Fourteen Public Folder mailboxes in Exchange 2016 to eighteen Public Folder mailboxes in Exchange Online. What could possibly go wrong….

Creating the new endpoint for the migrationbatch failed with an error saying “Multiple mailbox users match identity “PFMailbox1″. Specify a unique value” as shown in the following screenshot:

Or in plain text:

PS C:\PFScripts> $PfEndpoint = New-MigrationEndpoint -PublicFolder -Name PublicFolderEndpoint -RemoteServer $Source_RemoteServer -Credentials $Source_Credential
Multiple mailbox users match identity "Mailbox1". Specify a unique value.
    + CategoryInfo          : NotSpecified: (:) [New-MigrationEndpoint], MigrationPermanentException
    + FullyQualifiedErrorId : [Server=AM0PR09MB2402,RequestId=bccebc4f-d231-41d3-b8fe-a676311b3575,TimeStamp=22-8-2022 09:33:52] [FailureCategory=Cmdlet-MigrationPermanentException] 80069390,Microsoft.Exchange.Management.Migration. MigrationService.Endpoint.NewMigrationEndpoint
    + PSComputerName        : outlook.office365.com
PS C:\PFScripts>

This is strange since there’s no mailbox user named “Mailbox1”, there’s only one Public Folder mailbox named “Mailbox1” and that is holding the Public Folder hierarchy. But, last week I have been preparing the Public Folder migration, and one step was to create a Public Folder mailbox, just to see if it would work. I deleted the Public Folder mailbox, but it is a soft deleted state, and this conflicts with the creation of the Public Folder migration endpoint.

To find the Public Folder mailboxes including the soft deleted mailboxes, execute the following command against Exchange Online:

PS C:\PFScripts> Get-Recipient  -IncludeSoftDeletedRecipients -RecipientTypeDetails publicfoldermailbox | fl Name, OrganizationalUnit, DistinguishedName, ExchangeGuid

It will return a list of Public Folder mailboxes, including the ones in a soft deleted state. Clearly visible in the following screenshot is the soft deleted Public Folder mailbox:

Use the following command against Exchange Online to remove the soft deleted Public Folder mailbox:

[PS] C:\> Remove-Mailbox -PublicFolder "<ExchangeGuid>" -PermanentlyDelete

Wait some time for replication to happen in EXODS and try again to create the Public Folder migration endpoint. This time it succeeded.

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