Receiving mail servers, especially large mailbox providers, impose their own per-sender connection and message-rate limits, and exceeding them results in temporary rejections (soft bounces) or connection drops rather than outright blocking, at least initially.
A sending MTA throttles proactively, pacing delivery to each destination domain based on its observed tolerance, rather than sending as fast as the network allows and reacting to rejections after the fact. This matters most during warm-up, when tolerances are lowest.
Throttling is per-destination-domain, not a single global rate: a sender can be well within Gmail's tolerance while simultaneously being throttled by a smaller provider with stricter limits, in the same send.