SMTP error code 421 4.4.2 Message submission rate for this client has exceeded the configured limit

В powershell выполнить

get-receiveconnector | ft name,messageratelimit

set-receiveconnector -identity "Client Proxy Ваш сервер" -MessageRateLimit Unlimited

https://www.servolutions.com/support/articles/messagesubmissionrate.htm

https://social.technet.microsoft.com/Forums/ru-RU/d72c31a6-33f4-43f2-af0f-5b4712be1ea1/-receive-connector?forum=exchange2013ru