The S/MIME Email Object Algorithm provides the details of the internal format of a message and describes the mapping between the internal format and the Internet e-mail message format for two specific classes of Internet e-mail messages: opaque-signed or encrypted messages, and clear-signed messages, as described in [RFC5751].
When the server receives an Internet e-mail message, it maps the message to an internal format known as the Message object format. Similarly, when the client submits an e-mail message by way of the server, the server maps the message from its internal format to the Internet e-mail message format for sending. Also, in cases where protocols supported by the server allow saving or reading e-mail messages in the Internet e-mail message format, a similar mapping is required to and/or from the internal format. For more information about the mapping between the internal format and the Internet e-mail message format for other classes of messages, see [MS-OXCMAIL].
Sections 1.6 and 2 of this specification are normative. All other sections and examples in this specification are informative.