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

TRDx2 141 Reputation points
2021-05-24T22:06:41.407+00:00

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?

Exchange Server Management
Exchange Server Management
Exchange Server: A family of Microsoft client/server messaging and collaboration software.Management: The act or process of organizing, handling, directing or controlling something.
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  1. TRDx2 141 Reputation points
    2021-06-07T11:23:52.06+00:00

    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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  1. KyleXu-MSFT 26,211 Reputation points
    2021-05-25T06:31:08.363+00:00

    @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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