Many attacks, scams, and malware threats are based on or spread through emails nowadays. Although people have been fighting against them with technical and legal measures for many years, the situation is not mitigated but seems getting worse and worse, especially in the era of mobile computing. We attribute this to lack of end-to-end security solutions for emails in current Internet infrastructure. In this paper, we propose an add-on end-to-end solution for email security. It is based on idea

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... trust chain from sender to receiver, which spans multiple domains and organizations without requirement of a uniform platform. The solution is add-on, which means that it can be implemented on top of existing protocols as an optional component, without replacing email servers and routers in the Internet infrastructure. The solution provides end-to-end authentication and integrity for its users, which is hard to achieve in existing works.