In outlook express, outlook, and other mail clients the file that stores your email is portable if someone knows how to find it. In otherwords you can copy it and take it home with out even opening the email program.
Getting everyone that mails me to encrypt the mail is not a possibilty, for the most part getting any of them to would be painful task.
Are there any programs that offer the function of encrypting the mail after it downloaded? So that the folders the mail are stored in would be encrypted and nonviewable.