Certified e-mail system guarantees that recipient will get mail content if and only if mall originator receives a receipt. Unlike previous schemes, Optimistic protocols recently published generate a receipt when it meets the condition that the mail content can be accessed by recliner at any time. So originator cannot assure the delivery of e-mail although he can get a receipt. In this paper, we show some flaws in optimistic protocols and propose improved schemes using delivery deadline. Modified protocols guarantee proof-of-receipt and eliminate the problem mentioned above. Furthermore, proposed modification technique can be applicable to most optimistic protocols and is efficient in the sense that modified schemes do not increase the number of messages.