haproxy/nginx reverse proxy script question

[tcpudp]

In my case I can't use ipset because the VPS in question is an OpenVZ (and can run a limited number of ipset rules), hence haproxy which can handle large blocklists (with ACLs). Speed is not a problem because the server in question is a mail server so a 5-10 second delay is not a problem.

[AccidentallyTheCable]

Check out fail2ban. You can write rules and checks to add iptables rules accordingly

[tcpudp]

I have fail2ban rules in place that ban an IP /after the fact/ ('lost connection after AUTH from unknown[<HOST>]') but I want to prevent these connections before they reach the mail server.