One of our client migrating their MDaemon Mail Server into Zimbra Mail Server. Due to their poor setup on MDaemon mail server, their domain become DDOS Attack, thousand incoming spam mail from various IP and various unknown account with their domain as target. Zimbra successfully reject all spam by using b.barracudacentral.org and zen.spamhaus.org even before going to queue, but sometimes, all incoming thread full with DDOS.
Is there any advise to prevent DDOS attack, both within Zimbra, OS or using anti spam/anti virus/UTM appliance? Any advise highly appreciate, especially based on real experience.