"421 4.4.2 Message submission rate for this client has exceeded the configured limit" error message from Exchange

This error message indicates that the MessageRateLimit parameter in your Exchange receive connector is set to a very low limit.

├Łou can change this limit using these Exchange Management SHELL commands:

First get a list of receive connectors and their MessageRateLimits:

get-receiveconnector | ft name,messageratelimit

On our own server the output looks like this:

Identity : WIN-DKVEC8768RS\Default WIN-DKVEC8768RS
MessageRateLimit : 100

Identity : WIN-DKVEC8768RS\Client WIN-DKVEC8768RS
MessageRateLimit : 100

Identity : WIN-DKVEC8768RS\local apps relay
MessageRateLimit : unlimited

Now select the receive connector that is active on your own server. Most of the time it will be the "Default YOURSERVERNAME" connector. To change the maximum accepted header size use this command:

set-receiveconnector -identity Default* -MessageRateLimit 200

If you are unsure about the active receive connector just do this with all receive connectors, one after the other

You'll see the value changed when you do another get-receiveconnector like above again.