Fast Message Retrieval for IMAP4 Users
Topic Last Modified: 2007-06-20
Internet messaging client computers that access Microsoft Exchange Server by using Internet Message Access Protocol version 4 (IMAP4) use the streaming store (.stm file) for reading and writing mailbox data that is stored as MIME content. If you use the Move Mailbox functionality in Exchange System Manager or Active Directory Users and Computers to move a mailbox that is accessed by an IMAP4 client, the data in the moved mailbox is promoted into the Extensible Storage Engine store (.edb file), and the content is converted from MIME content to MAPI content.
When the IMAP4 client logs on, exact message sizes are calculated, and MAPI-to-MIME conversion of the mailbox data occurs in memory and on the disk of the server. The conversion on disk occurs in the Microsoft Windows TMP folder, which is usually not on a disk that has many spindles. The disk cannot handle the large number of I/O requests caused by a conversion of so many messages. Therefore, a user may experience long delays (up to several minutes) when the user does something as simple as switching between messages on the client. This behavior occurs because the disk that the TMP folder is located on cannot keep up with all the disk activity that Exchange generates to convert messages. One way to avoid the adverse effects of this behavior is to move the TMP folder to a disk that has a high-performance caching controller connected to it and enough spindles to handle the conversions. For more information about how to move the TMP folder, see How to Move the TEMP and TMP Directories.
In extreme cases, very large temporary files may be created on the Exchange server when this behavior occurs. In some cases, without correct planning or available server resources, the conversion from MIME to MAPI may also cause system latencies. This may cause IMAP4 clients to experience long delays during logon because of the requirement for exact message size calculation. You can improve performance by enabling fast message retrieval for IMAP4 clients.
For more information about how to enable fast message retrieval for IMAP4 clients, see How to Enable Fast Message Retrieval for an IMAP4 Virtual Server.
For more information about latencies during MAPI to MIME conversion, see Microsoft Knowledge Base article 317722, Client latencies occur when Exchange 2000 Converts Mail from MAPI to MIME format.