Mailchimp is by far the largest provider for newsletters. Adding Mailchimp to your comment section is a no-brainer and is an easy way to get more subscribers through comments.
Mailchimp on the Front-end
With Mailchimp enabled in Comment Edit Pro, you'll see a checkbox just above the submit button.
Mailchimp Checkbox on the Comment Form
Enabling Mailchimp
You'll find Mailchimp in the Integrations tab in Comment Edit Pro. If you're setting it up for the first time, you'll see an enable toggle switch.
Enable Mailchimp Toggle Switch
Setting the Mailchimp API Key
Once Mailchimp is enabled, you can enter your Mailchimp API key.