How To Add a Captcha To Subscription Forms

MailPoet started supporting reCaptcha since version 2.8. 

If you want to enable this feature, just go to the Settings page > Advanced tab > and toggle the "Enable reCaptcha" option. You'll need to sign up for an API key pair at  reCaptcha's website and add your keys in the two input fields below the option toggle:

This option is global, which means, once activated, all your subscription forms will contain a reCaptcha validation field after you activate this feature.