Agent Set Throttle

Set the BU's send-rate ceilings.

No confirmation. Throttle bounds how FAST mail goes out, never whether it goes out or to whom: every recipient this policy defers is a recipient the send was already going to reach. Lowering it is strictly safe. Raising it risks a reputation event, but a recoverable one -- lower it again and the rate comes down. That is the line between this and the warmup ramp, which has no agent write surface precisely because its damage does not reverse.