Each time an email address is entered into the email field on a HubSpot form, the address is immediately validated by HubSpot. This functionality is built into all HubSpot forms and does not need to be enabled. If the email address is not valid (does not exist, is misspelled, etc.), the user will not be able to submit on the form.
There are two methods to provide additional protection against spam form submissions:
CAPTCHA gives you an added layer of protection against spam form submissions. It adds a step to your form, requiring visitors to verify that they are not a robot before submitting.
Block free email providers or specific domains (Marketing Hub Starter, Basic, Professional, and Enterprise only)
In addition to enabling CAPTCHA, you may also want to block free email providers or specific domains on your form to encourage your visitors to provide high-quality business email addresses.