Email::prepare_emails()

	public function prepare_emails() {
		global $wpdb;

		if ( empty( $this->args['users'] ) ) {
			return;
		}

		$user_ids = [];

		foreach ( $this->args['users'] as $k => $id ) {
			if ( is_email( $id ) ) {
				$this->add_email( $id );
			} elseif ( is_numeric( $id ) ) {
				$user_ids[] = (int) $id;
			}
		}

		$ids_str = esc_sql( sanitize_comma_delimited( $user_ids ) );

		if ( empty( $ids_str ) ) {
			return;
		}

		$emails = $wpdb->get_col( "SELECT user_email FROM {$wpdb->users} WHERE ID IN ({$ids_str})" );

		foreach ( $emails as $email ) {
			$this->add_email( $email );
		}
	}