Email is today's important communication tool, but it is also one of the main ways of cyber attack. As a result of certificates mistakenly issued by CA agency, man-in-the-middle downgrade attack, and the proposal of DNS-based Authentication of Named Entities (DANE), new progress has been made on the improvement of the current email protocol and the security of email. This study combs the widely used email protocol from the point of view of email encryption and verification, analyzes its advantages and disadvantages, summarizes the latest research progress of email protocols and the improvement of current email protocol, and proposes a secure email system architecture based on DANE. Finally, the development direction of DANE-based email system is summarized and prospected.