Add BlockedEmail, to block signups based on email. Track stats of how many times each email address is blocked, and last time it was blocked. Move email validation out of User model and into EmailValidator. Signup form remembers which email addresses have failed and shows validation error on email field.

lib/validators/email_validator.rb

@@ -0,0 +1,24 @@
+class EmailValidator < ActiveModel::EachValidator
+
+  def validate_each(record, attribute, value)
+    if (setting = SiteSetting.email_domains_whitelist).present?
+      unless email_in_restriction_setting?(setting, value)
+        record.errors.add(attribute, I18n.t(:'user.email.not_allowed'))
+      end
+    elsif (setting = SiteSetting.email_domains_blacklist).present?
+      if email_in_restriction_setting?(setting, value)
+        record.errors.add(attribute, I18n.t(:'user.email.not_allowed'))
+      end
+    end
+    if record.errors[attribute].blank? and BlockedEmail.should_block?(value)
+      record.errors.add(attribute, I18n.t(:'user.email.blocked'))
+    end
+  end
+
+  def email_in_restriction_setting?(setting, value)
+    domains = setting.gsub('.', '\.')
+    regexp = Regexp.new("@(#{domains})", true)
+    value =~ regexp
+  end
+
+end