I was trying to research the landscape of these the other day — And by research, I mean light Googling and asking on Twitter. Weirdly, very little comes to mind when thinking about spam detection APIs. I mean some kind of URL endpoint, paid or not, where you can hit it with a block of text and whatever metadata it wants and it'll tell you if it's spam or not. Seems like something an absolute buttload of the internet could use and something companies of any size could monetize or offer free to show off their smart computer machines.
Akismet is the big kid on the block.
You might think of Akismet as a WordPress thing, and it is. It's an Automattic product and is perhaps primarily used as a WordPress plugin. I run that here on CSS-Tricks and it's blocked 1,989,326 so far.
After you get an API key, you can POST to a URL endpoint with all the data it needs and it'll respond
true if it's spam or
false if it's not.
To get better results over time, you can also submit content telling it if it's spam or ham (ham is the opposite of spam... good content).
Several folks mentioned Plino to me.
There is a lot to like here, like the fact that it's free and returns a JSON response like you might be used to in development. There is the fancy buzzword "Machine Learning" being used here, too. It makes me think that with lots of people using this, it'll get smarter and smarter as it goes. But there is no way to submit ham/spam, so I'm not sure that's really the case.
There is other stuff that makes me nervous. It's clearly on Heroku which is kinda expensive at scale, and so with no pricing model it seems like it could go away anytime. Sorta feels like a fun-but-abandoned side project. Last commit was two years ago, as I write.
OOPSpam looks super similar to Plino, but has a pricing model, which is nice. They publish their latency, which is over two seconds. I haven't compared that to the others so I have no idea if they are all that slow. Two seconds seems like a lot for an API call to me, but maybe it's not that big of a deal since it's an async submit?
CleanTalk has a clear pricing structure and appears to have plenty of customers, which is a plus to me. The website looks a little janky though, which makes me worry a little.
(Sorry if that's a little rude, but it's just mental math to me. Good design is one of the least expensive investments a company can make to increase trust, so companies that overlook it make me wonder.)
It looks like they have a variety of anti-spam solutions though, which is interesting. For example, you can ask an API to see if an IP, email, or domain is on a blacklist, which is a pretty raw way of blocking bad stuff, but useful for stuff like protecting against spam registrations (rather than just checking blocks of text). They also have a firewall solution, which is interesting for folks trying to block spam before it even touches their servers.
There are a couple out there that seem rather specific to testing emails. As in, testing your own emails before you send them to make sure they aren't considered spam by email services. Here are a couple I cam across while looking around: