1. A digital rights management method implemented in a system including a client computer and a digital rights management server (RMS server) for protecting electronic mails (emails), comprising: by the client computer: (a) receiving encrypted data representing an email, the email including a message and one or more attachments, the data having been encrypted by an email application of the client computer based on an original digital rights management (DRM) policy selected by a user; (b) obtaining from the user one or more DRM policies to be applied to the attachments of the email, each attachment corresponding to one specified DRM policy; and (c) transmitting the encrypted data representing the email, along with data specifying the DRM policy to be applied to each attachment, to the RMS server; by the RMS server: (d) decrypting the encrypted data representing the email, including the message and the one or more attachments; (e) applying digital rights management protection to each attachment which has been decrypted, based on the DRM policy corresponding to the attachment as specified in the data received from the client, to generate a protected document for each attachment; (f) re-composing a email document which includes as attachments the protected documents for the attachments generated in step (e); (g) applying digital rights management protection to the re-composed email based the original DRM policy; and (h) transmitting the re-composed email document to an email exchange server.