A non-repudiation service is used to generate, collect and maintain the evidence of event or action that has been announced, and make this evidence verified by arbitration in order to confirm whether the event has occurred. The non-repudiation protocol based on cryptology can offer non-repudiation service. In this paper a new non-repudiation protocol NRPUM for user agent (UA) and mail transport agent (MTA) is proposed. The NRPUM protocol data is encapsulated in SMTP and POP3 protocol, so as to implement the mail transport non-repudiation. Through validation and analysis, it is shown that the NRPUM can effectively provide non-repudiation between UA and MTA.