Ninja Forms problem

You have a problem with NF form validation. It detects html fields that contains emails (eg mailto links) as email fields and validates those.

if (!empty($value) && is_string($value) && preg_match('/@.+\./', $value) && !str_contains($value, "\n") && !str_contains($value, '\n')) {
// rest of the code
}

Your approach needs to be changed to:

if (!empty($value) && is_string($value) && preg_match('/@.+\./', $value) && !preg_match('/mailto:/i', $value)) {
  $ninjaForm = new Zerobounce_Email_Validator_Form_Public('ninjaforms', $form_data['id']); 
  // rest of the code
}

This should be safer to validate only email addresses (or use a better regex).