This book offers the first comprehensive overview of contemporary systems for secure electronic/digital postage. It introduces a taxonomy of electronic postage systems and explains their security risks and countermeasures. The underlying cryptographic mechanisms are introduced and explained, and the industrial-scale electronic postage systems existing worldwide are sorted out with respect to this taxonomy. The author also discusses privacy and anonymous mail, the state of standardization of electronic postage, and the process of security evaluation and testing of electronic postage systems.