What is rate limiting?

Rate limiting happens when a receiving server restricts how many messages an IP may send within a time frame. It protects the receiving system from overload and encourages new or unknown senders to move slowly. It is the harbor master raising a hand and asking you to slow your approach.

In technical terms, rate limiting appears as SMTP codes such as four two one or four five one which signal a temporary soft bounce. These codes tell the sending server to retry later.