public function isRoleBased(string $email): bool
    {
        $user = mb_strtolower(mb_strstr($email, '@', true));
        $user = str_replace(['-', '_', '.'], '', $user);

        if (!is_null($is = &$this->cacheKey(__FUNCTION__, $user))) {
            return $is; // Cached this already.
        }
        if (in_array($user, $this::EMAIL_ROLE_BASED_STRINGS, true)) {
            return $is = true;
        }
        foreach ($this::EMAIL_ROLE_BASED_REGEX_FRAGS as $_regex_frag) {
            if (preg_match('/^'.$_regex_frag.'$/ui', $user)) {
                return $is = true;
            }
        } // unset($_regex_frag); // Housekeeping.

        return $is = false;
    }