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

  • Type: Knowledgebase
  • Date changed: 2 years ago
Solution #00006733

Scope:
All Barracuda Spam Firewalls in use with a Microsoft Exchange 2010 server, all firmware versions.

Answer:

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. 

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