2.1.2 subject Field

Within the subject field, the subject-string field MUST contain the data from the "Subject" header of the original message. This header is specified in [RFC2822].

The value of the subject-string field can consist of characters outside the ASCII character set range, as specified in [RFC2045] and [RFC2046]. The MIME content-type header of the respective body part in which the Envelope-Part structure is embedded MUST specify the character set to use to interpret the value of the subject-string field in accordance with the MIME specifications [RFC2045] and [RFC2046].