It seems that you can apply for a free Spamhaus DQS subscription for non-commercial use. Otherwise, you will have to pay from $250 per year. (Lmao)

My quick fix was to replace Spamhaus Zen with bl.spamcorp.net as a real-time blackhole list, and my plan is to test it before using it in production for small sites with limited email usage.

What free blacklists are you using, and what can you recommend?