Hi Emily! We don't throttle the sending of your emails. We send everything at once and don't have a way to slow things down.
Some servers may restrict the rate at which they accept emails sent through us, saying, "Whoa, too many. I'm only letting 100 in right now." But that isn't something we're able to control from our end. Your IT Team might be able to change settings at the server level to allow more connections at a time.