i want to moderate comment though email. how to do ?

See if you can find a plugin to do so would be the easiest. There is a plugin that lets you post blog posts by email. Perhaps it could be adapted to moderate comments instead of posting posts. IMO, such a scheme would be inherently fragile since the email would need to be very carefully composed to ensure the server properly interprets your messages. The post posts by email plugin:
https://ww.wp.xz.cn/plugins/postie/