I've received the same type of email multiple times from DigitalOcean and its extremely frustrating.

With even the smallest of traffic spikes, DigitalOcean will detect it as a DDoS and immediately cut off your server for 3 hours.

If even a typical (< 100Mbit) broadband-cable connection hits your server with a spike of traffic for less than 5 minutes, your server will be taken offline for 3 hours minimum.

I've used multiple VPS providers and dedicated-server providers and DO is absolutely the worst when it comes to DDoS policy.