Envelope Elements

Many MIME headers that map directly to Message object properties have string values. Unless otherwise specified, the string values are copied directly. All string values SHOULD be converted to Unicode (UTF-16 form) before they are copied to property values, and where applicable, the decoding specified in [RFC2047] is applied before generating the Unicode characters.

If there are multiple instances of a header, MIME readers SHOULD use the first instance to set the value of the corresponding property but MAY<110> use the last instance instead. However, in the case of multiple recipient fields, MIME readers SHOULD combine the content of all instances to set the value of the corresponding property.