There’s little you can do if you never had such problems in the past. DDOS is hard to battle with; you could get away by moving your box to another provider, but the IP reputation and other misconfigurations & stuff might bite you back causing a huge pita.

We rely on Mailinabox, which has fail2ban but the server being on a DigitalOcean network, they claim to offer some kind of DDOS protection.