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Troubleshooter: Efficiently Migrating IMAP Users

We recently moved several mailboxes, most of which belonged to IMAP users. The migration took an unusually long time and left our EDB file bloated. Now, clients are complaining that access to their mailboxes is slow. How can we prevent this situation from happening during future moves?

Exchange 2000 Server uses two data files for message content: the STM file contains native-format Internet content, including MIME message parts, and the EDB file contains Messaging API (MAPI) property data, including message content for MAPI-submitted messages. When you use the Microsoft Management Console (MMC) Active Directory Users and Computers snap-in to move mailboxes, the utility promotes or copies all STM file data into the EDB file. This step explains why the resulting EDB file was so big and why your migration took so long: Most of the old mailbox data is now in the EDB file, and copying all that data takes time. Logging on to each client's mailbox also takes longer because their mail programs will request message sizes, which must be recalculated each time. To prevent these problems from occurring during future moves, you must perform two steps.

First, to reduce the amount of time each move takes, set up a striped or, better yet, mirrored stripe volume and edit the TMP and TEMP environment variables to point to the new volume. Striping gives you maximum I/O throughput, which is useful because each message that an IP client loads must be converted from MAPI to MIME format. The Microsoft article "XADM: Client Latencies Occur When Exchange 2000 Converts Mail from MAPI to MIME Format" ( describes why this works in greater detail. (Hint: Use Performance Monitor to watch the disk queue length and I/O rates to determine whether the temporary directory location is the bottleneck.)

Second, you can set a registry value to tell the POP and IMAP handlers to return approximate message sizes instead of exact values. Navigate to the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\POP3svc\Parameters registry subkey, add a value of type REG_DWORD named Compatibility, then assign it a hexidecimal format value of 0xfffffffe. Navigate to the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\IMAP4svc\Parameters registry subkey, and add the exact same value. After you complete these steps, clients will be able to log on and retrieve messages without waiting for the Store to perform an exact message-size calculation.

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