Maximum number of concurrent connections has exceeded a limit

Error "Service not available, closing transmission channel. The server response was: 4.3.2 The maximum number of concurrent connections has exceeded a limit, closing transmission channel"

At the same time the following error is logged on the Exchange server:

Log Name: Application

Source: MSExchangeTransport
Event ID: 1021
Task Category: SmtpReceive

Description:

Receive connector Relay for printers and applications rejected an incoming connection from IP address <<your IP>>. The maximum number of connections per source (20) for this connector has been reached by this source IP address.

This is a configurable limit on the Transport Service in Exchange which is set by default to 20. You can view this setting by using the following command in Exchange Management Shell:

Get-TransportServer | FT Name,MaxOutboundConnections,MaxPerDomainOutboundConnections

To raise the limit of 20 concurrent connections to (for example) 50 concurrent connections you can use the Set-TransportServer command in EMS:

Get-TransportServer | Set-TransportServer -MaxPerDomainOutboundConnections 50

However we got another error like the one above, we got The same error as below but it only applied to one receive connector, Not the whole organisation


Receive connector <name> rejected an incoming connection from IP address <ip> The maximum number of connections per source (20) for this connector has been reached by this source IP address.

This also means there is the same limit on the receive connector's as well, which needs updating.

Therefore the above fix would not work for us, we needed to check with this, which showed us the 20 limit....

Get-ReceiveConnector "Bear1\BearFlow" | fl MaxInboundConnectionPerSource

Then you can update it to 50 connections with this:

Set-ReceiveConnector "Bear1\BearFlow" -MaxInboundConnectionPerSource 50

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