How do I troubleshoot 421 4.3.2 The maximum number of concurrent connections has exceeded a limit, closing transmission channel?

When You get this response when connecting to Microsoft Exchange 2007-2010 server. 

 

   telnet 172.16.2.14 25
   Trying 172.16.2.14...
   Connected to 172.16.2.14 (172.16.2.14).
   Escape character is '^]'.
   421 4.3.2 The maximum number of concurrent connections has exceeded a limit, closing transmission channel
   Connection closed by foreign host.

 

This is a restrictions on the Receive Connector in Exchange 2007. You will need to configure the receive connector to allow MaxInboundConnectionPerSource to a desired number. The default value is 20. 

 

You can Change the "Maxinboundconnectionpersource" to 100 to receive more number of connections from the senders system 

 

Please try the command 

   Set-receiveconnector - identity "connector name" -MaxInboundconnectionPersource 100

 

Allowing more connections will fix the issue.

For Exchange 2016 you may need to run this command first
   Get-ReceiveConnector -Identity <connectorname> | fl MaxInboundConnectionPerSource

You can also reference the following Microsoft documentation

https://docs.microsoft.com/en-us/powershell/module/exchange/mail-flow/set-receiveconnector?view=exchange-ps