When you use the New-MailboxExportRequest cmdlet to export a mailbox as a .pst file in Microsoft Exchange Server 2010, the Modified Date of the mail items in the .pst file is changed to the date that you run the command.
Note The Modified Date of the mail items is changed even if the mailbox is set to Litigation Hold.
This behavior is by design. This is because it is important that forensic tools can identify when a mailbox was exported as a .pst file. Forensic tools are run on .pst files to collect modification information.