A method and system for transmission of digital content via e-mail with point of use digital rights management is disclosed. The secured access rights to the digital content may be customized for individual recipients by the sender, and may evolve over time. The access rights are enforced according to a time-dependent scheme. A key server is used to arbitrate session keys for the encrypted content, eliminating the requirement to exchange public keys prior to transmission of the digital content. During the entire process of transmitting and receiving e-mail messages and documents, the exchange of cryptographic keys remains totally transparent to the users of the system. Additionally, electronic documents may be digitally signed with authentication of the signature.