A Thank You goes a long way. Thank Me Later is a small plugin for WordPress that have the capability of automatically sending a Thank You note (email) to those visitors that leave a comment on your website.
With TML plugin not only you can say Thank You but you can also prompt them to further engage with your site.
This plugin is very configurable. You can customize multiple messages and set when to send them in different times, making the message seem more personalized and user-friendly.
Thank Me Later administration page:
Plugin default email message: