IMAP4 and POP3 performance is slow between front-end and back-end servers

In a Microsoft Exchange 2000 Server or Microsoft Exchange Server 2003 front-end/back-end server configuration, you may experience slow performance for Internet Message Access Protocol 4 (IMAP4) and Post Office Protocol 3 (POP3) message delivery between the front-end servers and the back-end servers, as well as between the front-end servers and the client computers.

This issue can be caused by network latency. By default, IMAP4 and POP3 front-end servers use 1,024 byte buffers to write to Transmission Control Protocol (TCP), which results in many small TCP packets. This may cause a performance degradation issue where there is high network latency.

To resolve this issue, modify the buffer size for IMAP4 messages and for POP3 messages in Exchange 2000 Server or Exchange Server 2003 on the front-end server. To do so, follow these steps.

Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall your operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.

1. Install the latest Microsoft Exchange 2000 service pack. The minimum service pack that is required for this is Microsoft Exchange 2000 Server Service Pack 2 (SP2). For more information about how to obtain the latest Exchange 2000 Server service pack, click the following article number to view the article in the Microsoft Knowledge Base:
   301378  (http://support.microsoft.com/kb/301378/ ) How to obtain the latest Exchange 2000 Server service pack
   Note If you use Exchange Server 2003, you do not have to install any service pack or hotfixes. You only have to enable the regedit entry as described in this procedure.
2. Click Start, click Run, type regedit in the Open box, and then click OK.
3. Do one or both of following:
   • To modify the buffer size for IMAP4 messages:
     1. Locate and then click the following registry subkey:
        HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\IMAP4Svc\Parameters
     2. Right-click Parameters, point to New, and then click DWORD Value.
     3. Name the new value, MaxBufferSize.
     4. Right-click MaxBufferSize, and then click Modify.
     5. Click Decimal, type the new buffer size (in bytes) in the Value data box, and then click OK.
        
        Important Do not set a buffer size to less than 1,024 bytes. This may cause the IMAP4 service to stop responding. Set the buffer size in multiples of 1,024. For example, 32 x 1,024 = 32,768. Test your front-end/back-end server configuration performance with the following buffer sizes:
        • 8,192 bytes
        • 16,384 bytes
        • 32,768 bytes
   • To modify the buffer size for POP3 messages:
     1. Locate and then click the following registry subkey:
        HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\POPSvc\Parameters
     2. Right-click Parameters, point to New, and then click DWORD Value.
     3. Name the new value, MaxBufferSize.
     4. Right-click MaxBufferSize, and then click Modify.
     5. Click Decimal, type the new buffer size (in bytes) in the Value data box, and then click OK.
        
        Important Do not set a buffer size to less than 1,024 bytes. This may cause the POP3 service to stop responding. Set the buffer size in multiples of 1,024. For example, 32 x 1,024 = 32,768. Test your front-end/back-end server configuration performance with the following buffer sizes:
        • 8,192 bytes
        • 16,384 bytes
        • 32,768 bytes
4. Quit Registry Editor

Microsoft recommends that you do not set the MaxBufferSize registry entry to a value greater than 32,768. However, you may notice increased performance with a buffer size value of 65,535, depending on your network conditions. A larger buffer size uses more memory on the Exchange 2000 Server or Exchange Server 2003 computer and may decrease the number of users who can access the front-end server. While there is no limit to the maximum value that you can use for the MaxBufferSize registry value, you may experience unstable performance and memory issues if you assign too great a value to this registry entry.