email validator: fix undeliverable? method always returning false.

`undeliverable?` calls `smtp_enabled?` which we forgot to define. It
swallowed the error so it didn't fail but it always returned false.

app/logical/email_validator.rb

@@ -63,6 +63,10 @@ module EmailValidator
     "gmx.us" => "gmx.net",
   }
 
+  def smtp_enabled?
+    Rails.env.production?
+  end
+
   def normalize(address)
     return nil unless address.count("@") == 1