Performance issues in POP3, IMAP4, and WebDAV clients

When the following client computers connect to Microsoft Exchange 2000 Server and Microsoft Exchange Server 2003, they fully use the streaming store (.stm) file to read and write data:

•	Internet protocol (IP) client computers that are using Internet Message Access Protocol 4 (IMAP4) or Post Office Protocol 3 (POP3).
•	WebDAV-based clients, such as Microsoft Entourage 2004

If you use the Move Mailbox functionality in Active Directory Users and Computers to move data, the data is moved into the .edb file. Therefore, clients that use POP3, IMAP4, and WebDAV protocols to access their mailboxes may experience performance issues after you move their mailboxes between databases or servers.

For clients that use POP3 or IMAP, after the user logs on, message sizes must be calculated and MAPI-to-MIME conversion occurs in memory and on the disk of the server. In large environments this calculation can take much time.

For clients that use WebDAV clients, such as Entourage 2004, severe performance issues on the server side may be caused when content is converted for large messages in the Inbox and in other folders. In extreme cases, very large temporary files may be created on the server that is running Exchange Server when this issue occurs in POP3, IMAP4, or WebDAV scenarios.

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Performance issues in WebDAV clients

In addition to the issues that were described in the previous section, WebDAV client users, such as Entourage or Outlook Web Access users, may experience other issues. In Entourage, messages may remain in the Outbox or the Drafts folder. Additionally, the following event may be logged in the Application log:

Event ID : 1000
Event Type : Error
Event Source : Application Error
Computer : Server_Name
Description : Faulting application w3wp.exe, version Version_Number, faulting module exprox.dll, version Version_Number, fault address 0x0002ce94

Outlook Web Access users will sometimes receive the following error in the Web browser:Additionally, when an Outlook Web Access user or an Entourage user accesses the Exchange server, Internet Information Services (IIS) may stop responding on the Exchange server. Then, you may receive the following error message:These issues occur when Outlook Web Access users or Entourage users download large attachments to the client computers that exceed the current message size limit. For example, attachments in older messages that exceed the current message size limit will cause these issues. Although users cannot send or receive large messages that exceed the current message size limit, the users can still access older messages that were delivered before the current message size limit was configured.

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If you are moving hundreds or thousands of mailboxes, you can use one or more of the following methods to prevent performance issues:

•	Make sure that the TMP and TEMP environment variables point to a very fast spindle set (possibly RAID 0+1). For stand-alone mailbox servers, we recommend that you adjust the system environment TMP and TEMP variables. For server clusters, we recommend that you configure the variables for the service account under which the server cluster is running.
•	Add the Compatibility registry value. This valueinforms the store to use approximate instead of exact calculations for message sizes. The following sections describe this procedure in more detail.
•	Add the MaxResponseSizeInBytes registry value in Exchange Server 2003.

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Add the Compatibility registry value for POP3 users

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:

The Compatibility registry value can be used to instruct the Exchange store to use approximate message sizes instead of calculating exact message sizes for POP3.

Add the Compatibility registry value for POP3 users

1.	Start Registry Editor.
2.	Locate and then click the following key in the registry: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\ POP3svc\Parameters
3.	On the Edit menu, click Add Value, and then add the following registry value: Value name: Compatibility Data type: REG_DWORD Radix: Hexadecimal Value data: 0xfffffffe
4.	Exit Registry Editor.

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Enable fast message retrieval for IMAP4 users

1.	Open the properties of the IMAP4 virtual server.
2.	On the General tab, click to select the Enable fast message retrieval check box, and then click OK.

For more information about this issue, click the following article number to view the article in the Microsoft Knowledge Base:

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Add the MaximumResponseSizeInBytes registry value in Exchange Server 2003 for WebDAV clients such as Entourage

The MaximumResponseSizeInBytes registry value helps allocate more memory to the IIS worker process when Entourage users are working with large messages.

When content conversion occurs in Exchange Server for large messages, IIS may run out of memory space when it tries to sustain the communication pipe between Exchange Server and the Entourage client. When you set a larger value for this registry key, you help IIS sustain the communication pipe and the content conversion process for large messages.

Content conversion happens only when such messages are accessed by users in Entourage. Therefore, the effect on performance is distributed over time.

Add the MaximumResponseSizeInBytes registry key to at least those Exchange Server 2003 front-end and back-end servers that Entourage users use to connect to their Exchange Server mailboxes. To do this, follow these steps.

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:

1.	Start Registry Editor.
2.	Locate and then click the following registry key: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeWEB\DAV Note By default, the DAV key is not in the registry. You will have to create it yourself.
3.	On the Edit menu, click Add Value, and then add the following registry value: Value name: MaximumResponseSizeInBytes Data type: REG_DWORD Radix: Decimal Value data: value Note To calculate a value for the Value data field, determine the size of the largest message that you want the users to be able to send and receive. For example, if you decide to limit message size to 50 megabytes (MB) and under, you would use the following calculation: 1024 * 1024 * 50 = 52428800 (Decimal) = 3200000 (Hexadecimal) Enter the results of this calculation in the Value data field. In this example, you would enter 52428800 (Decimal) or 3200000 (Hexadecimal). Remember that the configurable range for this registry key is 1 MB to 1 gigabyte (GB). Better results occur if customers set it to a value of 500 MB or a higher value such as 800 MB. We recommend that you set the value back to the default value or to a lower value after the content conversion issue subsides in your organization. The default value of this registry key is 128 MB. We also recommend that you set correct messages limits and mailbox size limits on your Exchange server to prevent performance issues.
4.	Exit Registry Editor.

To put this registry key into effect under Internet Information Services (IIS), restart the Exchange Application Pool. Then, if an Entourage user tries to work with a message that is larger than the value in the Value data field, the user may experience performance issues. An oversize message may also cause performance issues on the server side. This may affect other users.

Important This registry key works only for Exchange Server 2003. It does not work for Exchange 2000 Server.

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