How so? First, as Hyeonu pointed out, the per-email overhead is very small (as always in systems, benchmark and you're often surprised about what the actual costs of an operation are). Also, the poster mentioned that an email is sent only when new users sign up.
For the sake of argument, though, let's assume we wanted to design an application that can support 10s of thousands of users signing up simultaneously. Would
tokio::spawn_blocking help here without additional effort? It's not clear. First of, the documentation doesn't describe what happens if the limit is exhausted (is the spawn rejected, or queued?) If the default limit is used, it's likely too low for the expected onslaught of users. If the limit is raised, you'll probably get similar performance behavior as for spawning individual threads. Whether there's a sweet spot where concurrency throttling has a measurable effect in the face of overload conditions is (in my own experiments) often difficult to determine and requires engineering effort you'd only invest if you actually expected and designed for such loads.