Not nessesarly...
You wont get them back in your OE, but unless you have purged your mailboxes, you can retrieve them as a plain text file.
Check where you have your email folders stored, open that folder. You can open your mailflies with an text editor like notepad or ultraedit (my fav) and read its contents. If you havent purged your mailboxes to clean up space, the deleted mails will still be in there and you can copy them out of there. There will be some control garbage in the file that OE uses, but the text will be readable, incl headers.
If you have purged your folders to save space, then there is nothing you can do and its gone forever.