You can recover deleted items from your hard drive (look what happened to Gary Glitter) including e-mails. Use something like Uneraser http://www.uneraser.com/
is this a personal machine or a machine at work? if the latter then there's a slim chance a copy of your email is still at the server (exchange / groupwise / lotus notes) which an administrator can restore for you - copies of emails are kept on the server for a configurable period - in case the email server shafts and needs rebuilding