I know many of us have used the fqrdns.pcre in Postfix's smtpd_client_restrictions for many years to help block "low hanging" spam. Long ago, after the project was abandoned by Stan H, I adopted it and moved it to GitHub:
One of Stan's principles with this helpful file was to avoid false positives. So I always struggled with requests to add more strict rules because I didn't want to increase that risk. For the past 18 months or so, the "development" branch of the project has been split into three separate files, which progressively block more spam and misconfigured mailers. The basic fqrdns.pcre file is still in the original location, and still aims for zero false positives. The fqrdns-plus.pcre file adds a few rules, but still has a very low chance of false positives. The fqrdns-max.pcre file adds a few more, but will block misconfigured mailers which may be attempting to deliver otherwise legit mail. How strict you choose to be is up to you, and you can use one, two, or all three files in conjunction to achieve your goals. You should (as stated in the README) list them in your smtpd_client_restrictions from most to least aggressive.
After 18 months of testing, I've moved the three-tiered approach to the "master" branch. Thank you to the many on this list who have helped test this three-tier approach. For those who have been grabbing the fqrdns.pcre file from the GitHub master repo, you probably won't see much (if any) difference in your spam blocking. For those who want to add additional tiers, the instructions are in the README.
Please feel free to open issues on GitHub for feedback or suggestions. I figured it was worth mentioning here on the Postfix list because this is where the file originated, and I know a lot of us have used it for a long time. These files, combined with Postscreen, block a massive amount of incoming junk and make life for my Postfix servers much easier. :)