Which of your marketing programs produces the greatest ROI as a percentage of sales? For most mailers, the answer is the house file e-mail program. Customers and prospects who have provided their e-mail address to you are a gold mine. This is a quantifiable fact that few marketers can argue. That fact makes the disparity between the return from a house e-mail program and the effort direct marketers put into building their e-mail list difficult to reconcile.