In our case, attacker simply targeted stratum IPs. There is no such service as CloudFlare for stratum and no ISP/provider can do null route of UDP therefore you cannot really fight 300gbps UDP flood.
It is not so hard to protect web server. You can move mail server to another server/IP.
So, how come someone protect his mail server ? I never heard Gmail to be down due to DDOS. There must be some way to hide the Mail Server IP as well...