All of us today, or at least most of us lead Internet lives. We are habituated to sending emails daily. All you need to do while sending an email is to add signature message to "Donate Blood".
Just by adding a signature line Get Your FREE Email ID at a free email service became number one. The idea was every time a message was sent, the awareness of Hotmail increased and they received maximum number of subscribers in the shortest possible time. The point being, if Hotmail could do it, why can't we? After all, our mission is to pass the "Donate Blood" message to maximum number of people in shortest possible time.