code: public function message_id_header( $headers, $iac_options, $item_ID ) {
  
  		$type = ( 'iac_comment_headers' == current_filter() )
  			? 'comment'
  			: 'post';
  
  		$item = ( 'post' == $type )
  			? get_post( $item_ID )
  			: get_comment( $item_ID );
  
  		$headers[ 'Message-ID' ] =  '<' . Iac_Mail_ID::generate_ID( $type, $item ) . '>';
  
  		return $headers;
  	}
docs: apply the ID header
  
  @param array $headers
  @param array $iac_options
  @param int $item_ID
  @return array