5.2.13 RFC-822 Message Specification: RFC-822 Section 4

The syntax shown for the Return-path line omits the possibility of a null return path, which is used to prevent looping of error notifications (see Section 5.3.3). The complete syntax is:

       return = "Return-path"  ":" route-addr
              / "Return-path"  ":" "<" ">"

The set of optional header fields is hereby expanded to include the Content-Type field defined in RFC-1049 [SMTP:7]. This field "allows mail reading systems to automatically identify the type of a structured message body and to process it for display accordingly". [SMTP:7] A User Agent MAY support this field.