Por favor, tente usar o comando Export-Mailbox em vez de New-MailboxExportRequest
Export-Mailbox -Identity <MailboxIdParameter> -TargetMailbox <MailboxIdParameter> -TargetFolder <Target_Folder_Name> -ContentKeywords "<String[]>" -AttachmentFilenames "<String[]>" -StartDate "<Month/Day/Year> <Time>" -RecipientKeywords <String[]>