Popups are broken, get Popup Zen.
Does it use the wp_mail() function to send mail?
Yes, if you have an SMTP plugin like Postman, Mailgun, or other mail plugin it will automatically use that to send mail.
How do I setup MailChimp?
First, add your API key under Popup Zen => Settings.
You can find your API key in your MailChimp account under Account => Extras => API Keys. Generate a new one if it doesn't exist.
Save your API key.
Next, in your popup, choose Mailchimp as the opt-in provider. Add your list ID. You can find this under Lists => Your List => Settings => List name and defaults. Look on the right side of the screen for List ID.
Copy/paste that into the MailChimp list ID field and save.
How do I find my ConvertKit form ID and API key?
Your API key is on your account page.
To get your form ID, visit Forms => your form. Look in the browser address bar, it should be something like this:
That number is the form ID, in this case it's 445667. Enter that number as the ConvertKit form ID.
How do I setup MailPoet?
Install MailPoet, version 3 or later. Create a new popup, and select MailPoet as the email provider. Choose your list and save, new subscribers will be added to this.
Troubleshooting
Emails are not sending
The wp_mail() function is unreliable on many hosts. Install Postman or another SMTP plugin to use a more reliable mail service.