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;
	}