code: private function get_mail_to_chunk( $to, $send_next_group = array() ) {
  
  		$object_id   = $this->options[ 'static_options' ][ 'object' ][ 'id' ];
  		$object_type = $this->options[ 'static_options' ][ 'object' ][ 'type' ];
  
  		if ( empty( $send_next_group ) ) {
  			// split total remaining recipient list in lists of smaller size
  			$chunk_size      = $this->options[ 'static_options' ][ 'mail_to_chunking' ][ 'chunksize' ];
  			$send_next_group = array_chunk( $to, $chunk_size );
  		}
  
  		/**
  		 * Group of recipients
  		 *
  		 * @param array  $send_next_group (List of next recipients to process)
  		 * @param int    $object_id
  		 * @param string $object_type
  		 *
  		 * @return array
  		 */
  		$to = apply_filters( 'iac_email_address_chunk', array_shift( $send_next_group ), $object_id, $object_type );
  		$to = implode( ',', $to );
  
  		if ( ! empty( $send_next_group ) ) {
  			wp_schedule_single_event(
  				time() + $this->options[ 'static_options' ][ 'schedule_interval' ],
  				'iac_schedule_send_chunks',
  				array( $object_id, $object_type, $send_next_group )
  			);
  		}
  
  		return $to;
  	}
docs: Modulate email addresses for sending,
  register schedule event and convert address array in chunks
  
  @since 0.0.7 (2016.04.09)
  
  @param array $to (List of total remaining recipients)
  @param array $send_next_group (List of the next recipients groups)
  
  @return string