目前網際網路上的電子郵件是目前傳遞訊息的熱門方法之一。企業因導入相關的知識文件管理與保護系統使企業內部文件受到安全保障及保護，但也使得以電子郵件寄送至企業外部時針因夾帶受保護的檔案，會使對方收件人無法直接閱覽，造成使用上的不便。本系統架構將以成為SMTP其中一個MTA節點的方式使既有的郵件系統能基於多階層白名單規則進行附件的存取控管及自動解除檔案保護，並針對一封同時擁有多個收件人的信件得以個別檢測收件人並確保相對應的存取控管、自動解密及維持收件人列表的一致性。;Currently, sending emails via the internet is the most popular method all over the world. To protect internal documents, most enterprises would deploy relevant Document Management and Protection systems. However, this in turn, creates a problem and an inconvenience for other external users as they are unable to open the emails with the protected files directly. In this thesis, our system architecture will enable the existing mail system to perform access control and automatic decryption on attachments based on multi-level whitelist rules in a way that becomes one of the MTA mail relay nodes of SMTP. For an email that has multiple recipients at the same time, it is possible to individually detect the recipient and ensure the corresponding access control and automatic decryption of the attachments are consistent with the recipient list.