It's what I mostly did when I used to sell proxy email blasts. I searched for the keyword proxy on DP and just PM all the people that once bought an email blast from someone. Ofcourse I told them that it wasn't spam, they can go ahead and remove it right away and sorry if I wasted their time. But I'm sure that almost 80% gave a response and 3/4 of all people bought a spot
Works VERY good IMO.