You have to modify the sender's public-key certificate, make sure the sender would have a copy of that and modify the recipient's key as well, Jones says.

CNN: Technology - Security flaw discovered in Network Associates PGP software

Messages that appear in the priority box will initially be determined by an algorithm based on such as factors as the identity of the sender, key phrases in the message and whether the mail was sent to an individual or a large group.

CNN: Gmail looks to clear clutter with Priority Inbox

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

The MIT mathematicians, that is, sidestepped a weak spot that had plagued cryptographers ever since the days of the ancient Greeks: How do sender and receiver hand off a key without risking that their communication channel or their courier will be compromised in some way?

FORBES: I've got a secret