1.BY providing message recipients one-time access to a secure Web page that contains your message.Recipients get an email containing a link to secure webpage and direct readers to the page with the message, which is accessible only once. After that, the message is deleted.
2.The way most of these emails seem to work is by replacing the plain text of your email with a graphic image of the text, which is stored at the Web site’s server rather than in the email message itself.The original message just contains a link to this image. So user can view the image only and sender can delete the image whenever he wants
in the last case you can wipe the disk with all the emails every x hours and overwrite it (with present recuperation software it will take 8 tot 12 overwrites to have no trace whatsoever anywhere on the computer)