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

Introduction

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

Problem

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).

Solution

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 MAIL01.exchange.com Microsoft ESMTP MAIL Service ready at 
 Thu, 16 Jul 2009 07:09:42 -0500
250 MAIL01.exchange.com Hello [192.168.2.40]
mail from:<>
452 4.3.1 Insufficient system resources