Some mailboxes may be reported as failed in your mailbox move report, with the following error information:
Error occurred in the step: Opening source mailbox. Failed to open mailbox with error: ClassFactory cannot supply requested class, error code: -1056749262.
Check the properties of the account in Active Directory Users & Computers and you should find that the account is disabled. Exchange will not move a mailbox for a disabled user account. Your options here are to enable the account temporarily while you move the mailbox, or just delete the mailbox from your legacy Exchange server.
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