If you encounter the following warning when sending email via the mailRelay service it indicates you are exceeding the receive connector rate limit on your Exchange server.
421 4.4.2 Message submission rate for this client has exceeded the configured limit
To determine the current threshold run the following PowerShell:
Get-ReceiveConnector | ft Name, MessageRateLimit
To increase the threshold adjust the following PowerShell for your needs:
Set-Receiveconnector -identity <
Warning:
This article is provided for guidance only. Implementation of changes to your Exchange environment are at your own risk.