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