What is “rate control” by ISPs?

Rate control throttles incoming mail volume:

What it means:

ISPs limit how much mail they accept per time period. Excess messages are deferred or rejected. Not permanent rejection; just slow down.

Why ISPs use rate control:

Infrastructure protection: Prevent system overload. Behavior evaluation: Assess sender before accepting volume. Spam mitigation: Limit potential spam damage. Fair access: Prevent one sender from monopolizing.

Rate control signals:

421 codes with "rate" or "throttle" messages. Increasing deferrals during sends. Microsoft's RP-001 codes.

Response:

Slow your sending rate. Spread sends over longer periods. Improve reputation for higher limits. Respect throttling signals.

Rate control is the harbor master managing dock traffic. Too many ships at once creates chaos.