"421 4.4.2 Message submission rate for this client has exceeded the configured limit"

The MessageRateLimit on an Exchange receive connector is set too low for the volume POPcon is submitting.

Fix

First, find the active connector and its current limit:

get-receiveconnector | ft name,messageratelimit

Then increase it:

set-receiveconnector -identity Default* -MessageRateLimit 200

Re-run the first command to confirm the new value is applied. If uncertain which connector is active, apply the command to each connector shown.