Comment Mail requires a minimum of PHP 5.4+ to install and run.

Comment Mail is also compatible with PHP v7.0, you must be running PHP v7.0.9+. Starting with Comment Mail v161118, if you're running PHP v7 < 7.0.9 a warning is displayed in the WordPress Dashboard asking the site owner to upgrade to PHP v7.0.9 or higher. Note: While PHP v7.0.9 is adequate, PHP v7.0.10 is strongly recommended.

If you are running into issues with PHP Compatibility please open a GitHub issue here.