protected function getThrottleCacheKey(Exception $exception)
    {

        // if we haven't already set the cache key lets set it
        if ($this->throttleCacheKey == null) {
            // make up the cache key from a prefix, exception class, exception message, and exception code
            // with all special characters removed
            $this->throttleCacheKey = preg_replace(
                "/[^A-Za-z0-9]/",
                '',
                'laravelEmailException'.get_class($exception).$exception->getMessage().$exception->getCode()
            );
        }

        // return the cache key
        return $this->throttleCacheKey;
    }