With export-mailbox it’s possible to export a mailbox to, for example, a pst file. When using the parameter -DeleteContent normally the contact will be deleted after exporting the mail. In some cases this does not happen. It looks like to happen if a mailbox contains more then 4000 items
Microsoft knows about this issues and is busy creating a hotfix for this issue. This hotfix is not published at the moment but via the Technet forum you can find a KB articlenumber under which it will be published: kb960367. When you need the hotfix rightnow please contact CSS to get it.