Inbound Mail Flow Stops to Exchange 2013 Server

Over the past couple of weeks, I have performed a few transitions to Exchange 2013 and each have had a similar issue with inbound mail flow stopping, both oddly enough with a Barracuda spam firewall at the front end.  The issue occurs when mail flow is transitioned from a legacy server to a new Exchange 2013 server where mail flows initially but comes to a complete halt (queued on the Barracuda) after you think all is good.  When the Barracuda is repointed to the legacy server, inbound mail flow resumes its normal function.

What I have found out, in these specific scenarios, is that the default receive connector on the Exchange 2013 server(s) has to be modified.  The default receive connector has an initial limit of 2% for inbound connections from a single IP address for the MaxInboundConnectionPercentagePerSource setting and an initial setting for MaxInboundConnectionPerSource of 100 connections from a single IP address.  Unless you anticipate a very small amount of inbound email, I recommended changing one of these two settings.

To resolve the issue in both cases, I set “MaxInboundConnectionPercentagePerSource” to 100 percent on the default receive connector of the Exchange 2013 server(s).  The command you will need to run from the Exchange Management Shell (EMS) on a 2013 server is…

  • Set-ReceiveConnector -MaxInboundConnectionPercentagePerSource 100

After this change, you can restart the 2013 transport service and repoint the Barracuda to the 2013 server to resume normal mail flow.

For additional information about Exchange 2013 message throttling and receive connector settings, refer to these two TechNet articles…

I am certain there are probably alternate resolutions, therefore I welcome your comments and suggestions.

Good luck.

Todd (@oddytee)

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