If the Windows 8 Installation made a backup copy of the previous Windows directories, which is usually written to a directory called C:\Windows.old you may find the previous mail items in there.
If so, you could then try importing them into the new windows mail system and then take a look at the received items which should show the address sent to, which in this case would be your dads!
If you Google around for how to import previous mail items from Windows.old you should find some appropriate info. For example,
http://answers.microsoft.com/en-us/windows/forum/windows_7-windows_install/retrieving-email-from-windowsold-folder/fe9544b8-9f3c-4d85-8201-c0f3daaadb68
Another option is to explore if there is anyone else such as yourself that has previously received a mail message from your Dad, in which case again you could check the received from mail address.