Maximum number of concurrent connections has exceeded a limit

Customer is running a web application and this web application is able to send SMTP messages, for example after a new user registration or a ‘forgot my password’ option that sends out a link for resetting a password.

This application is not always able to send out messages, when it’s not able to the following error is logged on the web server:

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

Date: 25-10-2013 9:15:11

Event ID: 1021

Task Category: SmtpReceive

Level: Warning

Keywords: Classic

User: N/A

Computer: e2010-ht02

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.

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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 MaxOutboundConnections,MaxPerDomainOutboundConnections

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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

This way the web application is able to send out (large amounts of) SMTP messages again.

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