property Message.Header (Name as String) as String
Retrieves the header field attributes giving the name for the header field.

TypeDescription
Name as String A string expression that defines the name of the header field, or empty to get the entire message's header.
String A string expression that indicates the header field attributes.

Is the user passes empty string to Header property, it retrieves the entire message's header.  See the RFC 2045 for the list of the header field names.