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SMTP Service Tuning

The following is tip #9 from "25 Exchange 2003 Tips in 25 minutes"

The following is Tip #9 from "25 Exchange 2003 Tips in 25 minutes." This content is excerpted from Scott Schnoll's book, "Microsoft Exchange Server 2003 Distilled," brought to you by © (2004) Pearson Education, Inc. Publishing as Addison-Wesley Professional. Return to the main page for more tips on this topic.

On busy Exchange 2000 systems that sustained large SMTP message queues (e.g., an average of 1,000 or more), performance constraints were encountered because of a default setting on the SMTP service of a maximum of 1,000 file handles. Each time the SMTP transport stack on an Exchange 2000 (or Exchange 2003) server receives a message, it is streamed out to the file system, where it waits to be routed to its destination. To write it to the file system, the SMTP transport stack obtains a file handle and then passes the message into that handle. Because Exchange 2000 defaulted to a maximum of 1,000 file handles, the SMTP service could write only 1,000 simultaneous messages to the file system.

To improve performance for these large systems, three registry entries were often simultaneously adjusted to increase the maximum number of file handles that could be opened by the SMTP service (so that more messages could be processed) and to decrease the number of open file handles for the installable file system, another Exchange component (to avoid running out of memory when the queue is large). These registry values, which did not exist by default and therefore needed to be added manually, are listed here.

Location: HKLM\System\CurrentControlSet\Services\SMTPSVC\Queuing
Value: MsgHandleThreshold

Location: HKLM\System\CurrentControlSet\Services\SMTPSVC\Queuing
Value: MsgHandleAsyncThreshold

Location: HKLM\System\CurrentControlSet\Services\Inetinfo\Parameters
Value: FileCacheMaxHandles

The MsgHandleThreshold and MsgHandleAsyncThreshold entries would be set to the same value (some value greater than 1000), and the FileCacheMaxHandles entry would be reduced from 800 to 600.

Exchange 2003 dynamically calculates the appropriate settings for SMTP files handles, so these settings are no longer needed. Therefore, before upgrading any Exchange 2000 servers with these settings to Exchange 2003, you should delete the entries from the registry.

Get more "25 Exchange 2003 Tips in 25 minutes." Return to the main page.

About the author: Scott Schnoll, an Expert on, is an MCT, MCSA and a long-time Microsoft Most Valuable Professional (MVP).

In addition to writing "Microsoft Exchange Server 2003 Distilled," he is a co-author of the upcoming "Exchange 2003 Resource Kit from Microsoft Press" and lead author for "Exchange 2000 Server: The Complete Reference."

Scott has written numerous articles for Exchange & Outlook Magazine, and is a regular speaker at Microsoft conferences, including MEC and TechEd, as well as industry conferences such as Comdex and MCP TechMentor, where he covers topics such as Exchange, clustering, Internet Information Services and security.

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