Add-on plugin for gravity forms allowing you to specify email domains that will throw a validation error blocking them from submitting the form.
Email Blacklist Add-on for Gravity forms was built to help block submissions from users using generic or competitors email addresses to protect content displayed in a thank you message or confirmation email and remove non qualified leads from being collected. This plugin allows site admins to create a list of domains that if used in an email field on a Gravity Form will cause a validation error and block the person from submitting while using that email. A default email blacklist and validation message can be created to use across all forms with email field types. These default settings can be overridden on a per field basis.
Default settings can be added int he Gravity Forms Settings area. To add settings to an individual email field just add one to the form and got to the Advanced settings for the field.