The standard way to do this is to use a randomly generated destination address which can be reset by the user. If you want greater security than that you could

The standard way to do this is to use a randomly generated destination address which can be reset by the user. If you want greater security than that you could have the user include their 'secret word' in the subject line or body of the message; which will work as long as they can reset that too. Do not use this for anything where a single spoofed message could have dire financial consequences.