Trouble importing Sent messages from Outlook Express 6 to Windows Mail

So I exported all my old Outlook Express info to an external hard drive to import it all onto my new computer running windows 7 and Windows Mail.  When I try to import the Sent items (of which there are around 5400) a little box pops up showing importing the messages.  When it is done, I have an Imported Messages folder in my Storage Folders of Windows Mail, but the Imported Messages folder is empty.

Any ideas how I can get my old Outlook Express messages to successfully import to Windows Mail?

Thanks.