In Microsoft Office 365 Dedicated/ITAR, you receive an error message when you try to import PST files from Azure blob storage to Exchange Online Tenant. You may also experience this issue when you try to validate a mapping file.
This issue occurs if one of the following conditions is true:
There are spelling errors in the mapping file. This file maps the PST file in Azure to the correct mailbox in the EXO tenant.
The mapping file is too large to be validated.
There are duplicate objects in the EXO tenant. However, they can’t be seen by running commands such as get-mailbox or get-recipient.
The PST file in Azure is corrupted.
To resolve this issue, use one of the following methods, depending on the cause you have identified:
Check the mapping file to make sure that each mailbox can be seen by running get-mailbox against the Exchange Online environment on all mailboxes that are listed in the file.
There may be hidden objects in the EXO tenant because the objects haven't been completely removed from Azure. These objects can be found in the Azure Recycle Bin. Remove any users who may conflict with the user whose mailbox is being imported from the Recycle Bin in Azure. To do this, connect to Azure PowerShell, and then run the following command: