Personally I would hate to receive email like this, if I viewed the email on my phone I won't be able to view it on my work computer. If I'm working from home the next day I won't be able to view the message (ever again!). If I want to view the email in several months time I'm assuming it's highly unlikely I'll be able to.
Maybe there's a market for this, and if so good for you, but my honest feeling is that any desire for such a 'protection' is better solved by either talking to the person on the phone or in person, or getting them to sign an NDA, or if the recipient is so untrustworthy then don't engage with them.
Maybe there's a market for this, and if so good for you, but my honest feeling is that any desire for such a 'protection' is better solved by either talking to the person on the phone or in person, or getting them to sign an NDA, or if the recipient is so untrustworthy then don't engage with them.