Network Output Queue length beyond threshold

[This topic is intended to address a specific issue called out by the Exchange Server Analyzer Tool. You should apply it only to systems that have had the Exchange Server Analyzer Tool run against them and are experiencing that specific issue. The Exchange Server Analyzer Tool, available as a free download, remotely collects configuration data from each server in the topology and automatically analyzes the data. The resulting report details important configuration issues, potential problems, and nondefault product settings. By following these recommendations, you can achieve better performance, scalability, reliability, and uptime. For more information about the tool or to download the latest versions, see "Microsoft Exchange Analyzers" at]  

Topic Last Modified: 2007-02-13

The Microsoft® Exchange Server Analyzer Tool includes a performance data collection engine that is used to query performance counter objects on computers that are running Exchange 2000 Server or Exchange Server 2003. The performance data collection engine collects data from the Output Queue Length performance counter of the Network Interface performance object to analyze performance data.

The Output Queue Length performance counter shows the length of the output packet queue, in packets. The Exchange Server Analyzer reports the maximum value for the performance counter during the collection interval. If the average value exceeds 10, the Exchange Server Analyzer displays an error.

This error usually indicates that the server is experiencing periods of unresponsiveness, causing mail flow to slow down or stop. Additionally, Outlook clients who connect to this server may receive the RPC cancel Request dialog box.

The following are some of the possible causes of this error:


In certain circumstances, on a machine running the Microsoft Windows 2000 Server operating system, the Output Queue Length performance counter may consistently show a random value that is greater than zero. For more information about the circumstances in which the Output Queue Length performance might display the random value, and how to correct this, see Microsoft Knowledge Base article 834940, "System Monitor may display high values for the Output Queue Length performance counter in Windows 2000" (

  • A network interface card (NIC) on the computer cannot support the maximum bandwidth supported by your network infrastructure.

  • There is too much load on the server.

  • Viruses or SPAM are causing sudden elevated network usage patterns.

Take the following steps to resolve this error:

  • If NIC is does not support the maximum bandwidth of your network, upgrade the NIC to support a faster bandwidth.

  • Reduce the load on the server. You can do this by moving mailboxes to a server that has fewer mailboxes. If it is required, add an additional Exchange server to your environment.

  • If you suspect that the unusually high load is not caused by too many mailboxes on a server, try to identify the source of the unusually high load and stop it.

Consider the best practices in the following articles:

  • For information about network bandwidth considerations, see Network Performance in "Understanding Exchange Performance" in the Performance and Scalability Guide for Exchange Server 2003 (

  • For information about troubleshooting network problems, see "Ruling Out Network-Bound Problems" in Troubleshooting Microsoft Exchange Server 2003 Performance (