Encryption ensures that only I and the recipient can see the message.
Without e-mail certificate and the encryption and digital signature capabilities they bring, e-mail is about as good as a postcard. Postcards are easy to forge and certainly are not very private. An e-mail certificate is similar to the seal used by a medieval king. He holds the only seal so a letter sealed with it can be reasonable assured to have actually come from the king.
Right now, I am looking mostly at Comodo and Verisign.