Abstract: A certified e-mail protocol is about fair exchange of a message m for its receipt. Fairness is the most important property in exchange. We modify existing W-B signcryption scheme so that it can be used by certified e-mail protocols, and based on the modified scheme we propose a new certified e-mail protocol. The protocol makes use of an off-line and transparent trusted third party only in exceptional circumstances, and has lower computation overhead and better efficiency comparing related protocol.