Error sending SMTP "The maximum number of concurrent server connections has exceeded a per-source limit"
Question / Problem
Error messages when sending. "Error opening connection to SMTP server: The maximum number of concurrent server connections has exceeded a per-source limit, closing transmission channel."
Answer / Solution
This is a limitation of the receive connector of the MS Exchange Server. It only allows a certain number of simultaneous connections from a single IP. The default value is 20.
You can query the value for your receive connectors on the Exchange Powershell:
Get-ReceiveConnector -Identity <connectorname> | fl MaxInboundConnectionPerSource
To change the value you can use the following command:
Set-receiveconnector - identity "CONNECTOR_NAME" -MaxInboundconnectionPersource 100
Please restart the Microsoft Exchnage Transport service after changing the values.
Please also see MSDN: https://docs.microsoft.com/en-us/Exchange/mail-flow/message-rate-limits?redirectedfrom=MSDN&view=exchserver-2019#message-throttling-on-receive-connectors