Abuseipdb https://www.abuseipdb.com/ has an free api to check ip's and Report ip's who are abusive
I find it a very reliable source and as mdaemon postmasters, we could help reporting abusive ip's (automaticly after more than x unsuccesfull authentications from one ip) thus using the mechanism mdaemon already uses to block ip's
Thank you for your suggestion to integrate with the abuseIPDB. We will look into it for future versions.
Liu yon commented
Support to add this function, although you may not necessarily use that website, you can consider adding multiple options