Your last paragraph identifies the reason that running their own hardware makes sense for Fastmail. The demand for email is pretty constant. Everyone does roughly the same amount of emailing every day. Daily load is predictable, and growth is predictable.
If your load is very spiky, it might make more sense to use cloud. You pay more for the baseline, but if your spikes are big enough it can still be cheaper than provisioning your own hardware to handle the highest loads.
Of course there's also possibly a hybrid approach, you run your own hardware for base load and augment with cloud for spikes. But that's more complicated.
If your load is very spiky, it might make more sense to use cloud. You pay more for the baseline, but if your spikes are big enough it can still be cheaper than provisioning your own hardware to handle the highest loads.
Of course there's also possibly a hybrid approach, you run your own hardware for base load and augment with cloud for spikes. But that's more complicated.