@Gargron
Keep your money. We went that route with email and the best we ever came up with was heuristics (learning algorithms), but the spammers soon found ways around even that. The only way to stop spam is to not allow it in the first place. You achieve this by closing off any communications path that isn't controlled by whitelist or moderation. There is no other way. Maybe you can find one but I've been fighting these guys for 25 years now(*) including my work in this space for large commercial providers(**) and that's the conclusion I arrived at.

* Google "green card spam".
** Google "America Online". We blocked spammers. We applied learning algorithms using ~100 billion samples of known spam to seed the algorithms. We tracked them down and took them to court. We took their ISPs to court. And still they came.