WFFM - Custom validation class on form not being hit

Question

Using Sitecore 8.2

I have a form and I'm trying to add custom validation to the email field to block forbidden domains. I created the validation item in Sitecore and added it to my field:

and I created the corresponding class in my project:

namespace Example.Library.WebForms.Validators
{
    public class EmailValidation : FormCustomValidator
    {

        public EmailValidation()
        {
            this.ServerValidate += this.ValidateEmailAddress;
        }
        private void ValidateEmailAddress(object source, ServerValidateEventArgs args)
        {
            var blacklistFolder = Sitecore.Context.Database.GetItem(exampleConstants.exampleBlacklistFolder);
            var domains = blacklistFolder.Axes.GetDescendants().Select(x => x.GlassCast<IRestricted_Domain>());\

            var email = args.Value;

            foreach (var dom in domains)
            {
                var domain = dom.Domain;
                if (email.ToLower().EndsWith(domain.ToLower()))
                {
                    args.IsValid = false;
                }
            }
        }
    }
}

But it doesn't seem to be doing anything, and when I debug and attach to the ValidateEmailAddress method it never hits it. What am I missing?

EDIT: I changed the Assembly from Example.Library to Example.Library.dll but it's still not hitting it.