What does this service do that the USPS or I.T.Consultancy Ltd. PGP digital e-mail stamping service doesn do?
The USPS and I.T.Consultancy Ltd. systems are not intended for sensitive documents. The stamping service digitally signs your e-mail message very much like this service, but your e-mail message can be intercepted both on its way to the stamping service and from there to its destination, and its contents divulged. A second difference is that the digital signature certificate is not published and managed for public record by those services.
