ParentPart is a read-only integer property that indicates the ordinal position of the message part that is the owner of the content identified in the boundary marker at the top of the stack. 

ParentPart is used to support nested MIME parts where the boundary marker may be for a message part that makes up a "multipart/alternative", "multipart/related", or "multipart/mixed" content type. 

ParentPart returns the integer value that indicates the ordinal position of the message part, or -1 when there are no parent message parts stored in the internal list.