DDOS comes in several flavors. Most common one is just to dump too much traffic, so network can't handle it. Others are less common but can cause server to be

DDOS comes in several flavors. Most common one is just to dump too much traffic, so network can't handle it. Others are less common but can cause server to become unresponsive due to exhausting some other resource than network (cpu, file descriptors, huge amount of swap, ...) For the first type of attack you can't do much on your own you have to either work with your isp (or CDN for web traffic) If your server is still reachable over network but unresponsive that means you are suffering from second type. That you can usually do something about. You could put firewall with rate limiting (sometimes called traffic shaping), connections to your mail server. Setting up firewall is not something I can guide you in a comment so google for it. Good free one is pfsense