Exchange Back Pressure Error "4.3.1 insufficient system resources"

Many users have controversy regarding Backpressure Error, so I decided to put a detailed discussion here and make easy to understand what exactly back pressure is?, what it does? And how to resolve if an error 4.3.1 is occurred, so let us start from bottom and reached to top level of difficulty step by step.

What is Back Pressure?

Most of the Exchange Server administrator might have encountered an error message 4.3.1-insufficient system resources, before going deep about this error let us talk about Edge Transport Server and Hub transport Server which is used while discussing about error 4.3.1.

All the inbound and outbound internet mail flow are handled by Edge Transport Server that provide mail relay and smart host services for your Exchange organization. The agents running on Edge Server provide protection against viruses, spam and apply transport rule to control mail flow.

The Hub Transport server role handles all mail flow inside the organization, applies transport rules, applies journaling policies, and delivers messages to a recipient's mailbox. Messages that are received from the Internet are processed by the Edge Transport server before they're relayed to the Hub Transport server.

Now let's talk about error 4.3.1, the transportation services require some free disk space if the free disk space is not present then this error occurred. The transport services are responsible for monitoring system resources like required free disk space is available or not.

Edge transport server and Hub server together makes Transport Server if it runs slow and stops message submission, this situation is server overloaded state and is termed as Back pressure. The back pressure feature monitors transport queue, database in Exchange Server 2007. A detailed description of Back Pressure is given bellow.

Back pressure is used to monitor the system resource that communicate with Microsoft Exchange Transport Service that exist on Mailbox Server and Edge transport Servers. It checks all the required resources and ensures that all of them are in consistent state and ready to use and overcomes the condition of Exchange server overwhelmed and unavailable.

The transport server checks all the required resources before stablishing a new connection, if the resources are unavailable then it deny to stablishing a connection and prevent from overloading of transport service also prevents from error "4.3.1 insufficient system resources" and the establishment of connection will be resumed once the system resource utilization reduced to normal level.

The concept of Back Pressure comes when most of administrators were facing the error 452 4.3.1 insufficient system resources. Now you have a question that when this error is occurred so the answer is that when the back pressure level is set on High Level. Let's discuss different levels in which Back Pressure mechanism works.

Level in Back Pressure

Mostly three levels in which backpressure mechanism works and each level defines their significant meaning. Let's discuss meaning and working of all levels.

• Normal – All the resources are well occupied and is performing its role as intended, server accepts new connections and messages it ensure that the resource is not overused.
• Medium – The Backpressure is applied to Exchange Server in limited manner and resources are slightly overused. Typically internal email flow remains functional while email from external or non-Exchange sources will be rejected.
• High – This condition needs to apply full Back Pressure mechanism because the resources is severely overused. Flow of message stops and server deny to make new connections and rejects all new incoming mail from commands.

These transition level defines weather the resource pressure is increasing or decreasing. Here is not a static value to measure the different state of pressure it is defined according to use and access of resources.

How to Resolve Back Pressure Error "4.3.1 insufficient system resources"

When an end user unable to receive any inbound mail and displays the following error message "Return-path address rejected by mail.mydomain.com:25: 452 4.3.1 insufficient system resources"

It keeps returning this error message until the Back Pressure level set on low level, to resolve this error I am giving bellow you a best way.

To resolve this issue you need free space over 4 GB on the volume where the queue database is located and Configuration options for transport servers are saved in the XML file, EdgeTransport.exe.config, which is located in the following directory (\Exchange Server\Bin\).

If 4 GB free space is not present in database queue then you can simply move the database to another volume by following some simple steps that are given bellow.

1. Add the following key in the AppSettings section EdgeTransport.exe.config: <add key="QueueDatabasePath" value="D:\Queue\QueueDB" />
2. Restart the MSExchangeTransport service, and then the message submission will be resume.

Note: - the Back pressure feature that monitors pressure on resources and stop accepting new connections, if resource pressure is high then you can ovoid this situation. just turn OFF the resource pressure analyzer, To turn OFF the pressure analyzer follow the steps given bellow

1. Using Notepad, open the EdgeTransport.exe.config file from \Exchange Server\bin directory.
2. Add the following key+value pair: <add key="EnableResourceMonitoring" value="false" />
3. Save the file.
4. Restart the Microsoft Exchange Transport Service (MSExchangeTransport).

Conclusion: - I have discussed all the related aspects and given you a best possible way to resolve the Back Pressure error 4.3.1. If you have any query related this topic you can contact to our support team