No longer recommended
I am sorry, but I must withdraw my endorsement of Spam Interceptor. This service is too unreliable to be recommended. The idea of combining a filter with challenge-response is great, but the implementation is not good. A 1+ day email outage per month is intolerable. And good luck trying to reach them when the service is unavailable. If your phone or electricity were unavailable for 1+ day per month, you’d say that the service was as bad as a 3rd world nation. Why should we tolerate that in computers?