A full-featured e-mail system is used in both Internet-based and client-side (personal computer) forms. In each form, either basic e-mail service is provided to system subscribers or a secure, premium service with authentication, concealment, integrity, and non-repudiation functions for electronic messaging...http://www.google.ca/patents/US6356937?utm_source=gb-gplus-sharePatent US6356937 - Interoperable full-featured web-based and client-side e-mail system
