For many years we've been using CleanTalk blacklist API that helps fighting spam by checking IP addresses and email addresses. This API served us well, and filtered millions of spam messages over those years.
We have recently migrated to CleanTalk Anti-Spam API. This API improves even more the spam filtering capabilities by checking additional data points such as content of messages, user names, how fast a form is submitted and whether javascript was enabled in the browser.
With CleanTalk Anti-Spam we get even better results then before! Almost no spam message reaches the inbox.
Further more, Anti-Spam also us to report a submission as spam and this way help them improve the system and block similar messages in the future.