code: public function message_reference_headers( $headers, $iac_options, $item_ID ) {
  
  		$type = ( 'iac_comment_headers' == current_filter() )
  			? 'comment'
  			: 'post';
  		$parent = NULL;
  
  		switch ( $type ) {
  			case 'post' :
  				$item = get_post( $item_ID );
  				if ( 0 != $item->post_parent )
  					$parent = get_post( $post_parent );
  				break;
  			case 'comment' :
  				$item = get_comment( $item_ID );
  				if ( 0 != $item->comment_parent ) {
  					$parent = get_comment( $item->comment_parent );
  				} else {
  					$parent = get_post( $item->comment_post_ID );
  					$type   = 'post';
  				}
  				break;
  		}
  
  		if ( ! $parent )
  			return $headers;
  
  		$headers[ 'References' ]  = '<' . Iac_Mail_ID::generate_ID( $type, $parent ) . '>';
  		$headers[ 'In-Reply-To' ] = '<' . Iac_Mail_ID::generate_ID( $type, $parent ) . '>';
  
  		return $headers;
  	}
docs: apply reference headers
  
  @param array $headers
  @param array $iac_options
  @param int $item_ID
  @return array