Cisco Email Security Appliance

Why do I see the error "452 4.3.1 Insufficient system resources" when I deliver to my own domain?

Document ID: 117923

Updated: Jul 16, 2014

Contributed by Stephan Fiebrandt, Cisco TAC Engineer.



This document describes an error (soft bounce) message that you might receive during communication with nexthop Simple Mail Transfer Protocol (SMTP) Server(s).


The Delivery Connection ID (DCID) shows this message

452 4.3.1 Insufficient system resources

for the nexthop SMTP server. Messages are queued up on the Cisco Email Security Appliance (ESA).


Beginning with Microsoft Exchange 2007, a Back Pressure mode has been introduced. The most common reason why Microsoft Exchange will defer any message (except from its own domain) is because the disk space for the Exchange transport agent went below 4GB. The only solution is either to increase the disk space or move the queue to another partition with enough space.

Example SMTP conversation

helo exchange
220 Microsoft ESMTP MAIL Service ready at
Thu, 16 Jul 2009 07:09:42 -0500
250 Hello []
mail from:<>
452 4.3.1 Insufficient system resources
Updated: Jul 16, 2014
Document ID: 117923