MailPoet 3 Integration
If you aren't familiar with MailPoet, it is a free plugin that allows you to send newsletters and capture emails through your WordPress admin. If you don't want to use a service like MailChimp or Convertkit for your email marketing, this plugin is a great free solution.
Holler Box allows you to add email subscribers right to your MailPoet 3 lists. At the time of this writing MailPoet 3 is still in beta. In general you should not use beta plugins on a production site.
With MailPoet 3 active and at least 1 list, you can now create a new Holler Box using MailPoet for the opt-in. Choose MailPoet, select your list, and save.