Certified E-mail can be divided into an on-line protocol an optimistic protocol by the usage of a third party. We call this party “Delivery Authority”. An on-line protocol can realize send-and-forget. There is a drawback, however, that the cost to Delivery Authority becomes large, because users surely access Delivery Authority in the middle of a protocol. On the other hand, when an optimistic protocol is used, there is little cost to Delivery Authority because users access Delivery Authority only in case a problem occurs, however, send-and-forget is unrealizable. Each protocol may be inconvenient depending on the situation. In the conventional system, however, a sender selects which protocol to use depending on a sender’s convenience. In this paper, we propose the new system having both our proposed on-line protocol and optimistic protocol a receiver can choose freely. By this mechanism, a receiver can choose the usage of Delivery Authority freely according to that time after taking the necessity and the situation of Delivery Authority and the sender into consideration.