Envelope Elements

Many Message object properties that map to MIME headers have string values. Unless otherwise specified, the string values are simply copied from the property to the header. When MIME writers generate MIME header values, the encoding specified in [RFC2047] MUST be used to encode Unicode characters according to the conditions specified in section 1 of that document.

Likewise, unless otherwise specified, when a MIME message with a TNEF body part is being generated, all Message object properties SHOULD<21> be copied to the attMsgProps attribute, as specified in [MS-OXTNEF] section, of the TNEF body part, even if there is also a corresponding MIME header.