Hello Guys,
There are many types of attack exist and like Spamming, spoofing, bruteforce and DDOS.
1. Spamming and spoofing: You need to hardening Mail server to prevent these types of attack.
2. Bruteforce and DDOS: Firewall does a great deal to tackle this types of attack. For DDOS attack you also need hardware protection provided by the service provider. No software protection will be sufficient.
If your server is not fully managed server, then I recommend you hire a server admin or you may consider our server management plan (https://www.milesweb.com/server-infrastructure-management.php).
Enjoy !!