How can u permenantly delete a msg in gmail so that the other guy doen't recieve it?

by accident I sent something really important to a stranger and i want to delete it so its like that other person never got it .... how can one do that .... i noe there is an undo option that comes in a yellow strip but it goes away after a little while if u don't click it and i din't cuz i didn't noe at the time. wat do i do .. help guys????