When the recipient opens the e-mail, the message retrieves the decryption key from Disappearing's key server and it's displayed in decrypted form.

CNN: Make your e-mail disappear

But after the time limit specified by the sender, the decryption key is erased from Disappearing's key server, making the message unreadable and effectively destroying it, even if the recipient stored it.

CNN: Make your e-mail disappear