Throttling is a feature which will delay the accept command sent to connecting hosts when they request a connection to send email. It is a method to increase or reduce the amount of accepted incoming connections usually based on current message load. This will slow down the rate at which email messages arrive, and allow more time for a proxy server to process emails.
Throttling will help your email proxy process email, though not overloading the server with too many email messages which may burden it’s performance.
Wikipedia's guide to Bandwidth Throttling