Exchange 2003 running slowly

End user problem: Exchange 2003 is running slowly.
Symptoms: Ave disk queue length is huge, disk response time poor.

Red herring: It isn't the array, the controller, the OS, defragmentation, the stores, the Exchange installation. But by all means check them - I did.

The problem: If you are using Entourage (IMAP4 client for Apple Macs), that client requires MIME encoded mail. If you do a move mailbox from one store to another, you get all email converted BACK to MAPI. In other words, once you have moved users, their IMAP client has to convert it all from MIME to MAPI again which results in the disk thrashing and subsequent slowness of Exchange.

The more times you move mailboxes to counter this problem, the worse the problem becomes as Entourage/Exchange has to start again. By default, IIS converts these but IIS can run out of memory for large attachments and I've got lots of those.

Reg hack (terms and conditions apply, back up everything and only touch if you know what you are doing, do this at your peril etc, etc):

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeWEB
Create a new key called DAV
Add a new value in that new key:
Value name: MaximumResponseSizeInBytes
Data type: REG_DWORD
Radix: Decimal
Value data: value

Where value is the maximum msg size in BYTES. It has been suggested to set this to 1GB (1,073,741,824 decimal)

Then restart the Exchange Application Pool within IIS Manager.

Bearing in mind that some of the messages internally here are over 100MB, the IIS default was not entirely enough. Hopefully that has put that issue to bed.