Non-responses are probably my greatest pet peeve in the professional world.
We’re all adults, we can handle being told “Sorry there’s not enough work right now, check back in two months from now” or “Sorry, we can’t take on your project right now.” A total lack of response, even after several follow-ups, is unbelievably rude, and leaves the original sender in limbo.
Even if you get 500 emails a day, a quick “Sorry, no vacancies” email takes all of two seconds to write.