Interosa forces the recipients to display all of their messages within a special window that does not allow one to manipulate or print the contents unless the sender has granted specific permission. MailVault has a different form of similar protection built in. Of course, one could always photograph or videotape the screen. These methods are common in industrial espionage cases. But perhaps you shouldn't be sending an e-mail to someone if you're worried they may be taking a photo of it.
Common sense is always the best guide.
The advantage of these new systems is that if the expiration date is set relatively short, say five or 10 days, then the messages will have been rendered unreadable long before they would be discovered and used against the sender in litigation. That's where the real value lies. However, these purges should occur in the normal course of business. Otherwise, it could appear that the company is destroying the damaging evidence. Authentica is the only developer actually shipping product.