There is no absolute prohibition on small email services having an infrastructure that includes access to specialist services that assist in mitigating DDOS attacks. Indeed, at one point when their services were based at NYI, Fastmail seems to have had this pretty well under control. Of course, you do not get additional robustness in your systems for free, and even the largest, best protected systems on the planet can theoretically suffer from DOS if the attack is big enough and well designed enough.
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