The general quality of DDoS scrubbing services has dramatically improved in the last 10 years. I work for a large tech company and Silverline has protected us from 100G+ attacks.
Cloudflare is the most well-known, but there are lots of providers now that provides these level of service, from the old guys like Akamai's Prolexic to new ones like Imperva to tier-1 ISPs like Telia's.
Additionally, depending on the exact service, you can certainly firewall traffic - close to the source.
The specific problem here is that mail servers, since that is not the target of DDoS until now, which means that there are few companies who do provide mail exchange-specific DDoS protection, which means larger companies (Verizon/Yahoo, Microsoft/Outlook, Google/Gmail) just operate servers well beyond what they really need, and I don't think that they can just run to Cloudflare and violate their privacy promise in the process.
100gbps is basically trivial test for a new botnet