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...
Mail server is not so critical to protect. In worst case scenario, sending/receiving of mails will not work, which is not equal as crippling entire service. For attacker is usually not worth to attack only your mail server, because with such attack he doesn't gain much leverage.