How about revoking emails? Namely, the user sending something to @gmail.com could also send a special "mail revoke" message to revoke his sent mail. If the sender is also a gmail user, he could be offered a simple GUI option. To eliminate potential privacy concerns, the feedback would be either "revoke received" or "trying to revoke invalid message". Any other kind of message would let the sender know whether his mail was already read or not.
What would be the safe contents of such message? Something like SHA1(sender_address || SHA1(mail_body)) would be sufficient, although crude in the first iteration.
More importantly, mechanisms to securely "revoke" only own messages are available, why isn't such option already implemented?
Tags: google gmail