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421 4.3.2 The maximum number of concurrent server connections has exceeded a per-source limit, closing transmission channel

Today we ran into an interesting issue with our Exchange DAG. We have 2 datacenters with 2 DAG member in each datacenter. We have a file share witness in a third site and all sites are connected by MPLS connections. Today the MPLS circuit between the 2 datacenters started having intermittent connectivity issues. After about 20 minutes of very intermittent connectivity we stopped receiving emails from our outside email filtering service. when I attempted a telnet session to our mail servers I received the following message: 421 4.3.2 The maximum number of concurrent server connections has exceeded a per-source limit, closing transmission channel. Now I know I can adjust MaxInboundConnectionPerSource on the receive connectors. I think I would eventually exhaust whatever number I set it at though. My question is why would intermittent connectivity to DAG members that aren't doing anything cause inbound connections to be used up? What receive connectors do Exchange nodes use between each other? Would it be something for SafetyNet/ShadowCopy that would use a receiveconnector? If so which receive connector does it use?

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TRDx2 answered

The issue was with our default frontend receive connector. This connector receives email from our spam filtering company only. In the 4 years we have been using them we have never had more than 5 connections. I believe the issue was some how related to the connectivity issue between the 2 datacenters but I can't figure out why or how. I had opened a case with Microsoft but after working on this with them for a week was getting no where and closed the case. There was no clear indication as to what was holding the connections open. Once the circuit between sites was stabilized the connections sorted themselves out and everything was fine. I have since changed the number of connections allowed on this receive connectors from the defaults to 200. This will at least give me a buffer so that if it every happens again I can catch it before we start deferring emails.

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KyleXu-MSFT answered KyleXu-MSFT commented

@TRDx2

The Exchange DAG members using "MAPI" and "Replication" network to transfer data. They don't use receive connector.

For the issue that you are experiencing, it just caused by heavy load on mail flow. I would suggest you create a dedicated receiver connector for your outside email filtering service. And dedicated receiver connector for relay tools. They will could balance the load on your mail flow.

Improve the “MaxInboundconnectionPersource ” is also suggested.


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@TRDx2
I am writing here to confirm with you any update about this thread now?
If the above suggestion helps, please be free to mark it as an answer for helping more people.

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