Bounce backs and error codes for email can be very mysterious and misleading. To help with that we have started a new series on the blog dedicated to demystifying these mysteries. To read all of the blogs in this series, go here.

We see this type of error pretty often here at MxToolbox and so we wanted to post about it here. Customers Exchange servers can mistakenly react to normal mail flow and cause a disruption in service. The error that is posed is ‘4.3.1 Insufficient system resources’.

A feature called Back Pressure in Exchange 2007 can sometimes cause this error to be received when we try to deliver messages to the customer server. When Back Pressure detects overused resources the Exchange Server controls system resources to prevent them from being overwhelmed and it allows the delivery process for current messages to be worked out. All these processes are part of the Back Pressure feature which is responsible for monitoring certain Exchange Server 2007 resources.

The drive on which the Queue DB and logs are stored must have 4GB or more free space otherwise the server will apply back pressure and start slowing the flow of messages! The main database file is called mail.que and by default can be found here:

C:Program FilesMicrosoftExchange ServerTransportRolesdataQueue

Other helpful articles:
http://www.msexchange.org/articles_tutorials/exchange-server-2007/management-administration/understanding-back-pressure-feature-exchange-server-2007.html
http://exchangepedia.com/blog/2007/03/exchange-server-2007-transport-452-431.html