If the email message are still in the Outbox folder (i.e. they have not been sent yet), you can delete them before they get sent by selecting the messages from within Outlook Express and choosing Edit > Delete from the menu.
If you want to delete the sent messages (the ones that have already been sent), go to the Sent folder from within Outlook Express, select the messages and choose Edit > Delete.
If you want to delete the entire Outbox folder or the Sent folder, close Outlook Express, and do a file search for the files Outbox.dbx and Sent Items.dbx and rename them or delete them.
BTW, the .nk2 file you are referring to is a Microsoft Outlook file, and not an Outlook Express file. That's why you couldn't find it.
Hope this helps.
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