Performance in Outlook or in OWA decreases when you use IMAP4 to access the calendar folder in an Exchange Server 2010 SP1 environment

Symptoms

Consider the following scenario:

• You use Microsoft Outlook or you use Microsoft Outlook Web Access (OWA) to log on to a mailbox in a Microsoft Exchange Server 2010 Service Pack 1 (SP1) environment.

• You verify that the calendar logging feature is enabled.

• You use Internet Message Access Protocol version 4 (IMAP4) to access the calendar folder in the mailbox.

• At the same time, other users log on to the mailbox.

In this scenario, other users who log on to the mailbox by using Outlook or OWA experience adecrease inperformance. For example, an hourglass icon appears for several seconds in Outlook. Or, OWA remains stuck in the refreshing state.

Cause

This issue occurs because the IMAP4 service triggers an unnecessary copy on write (COW) action when a calendar cannot be converted to a MIME type. Then, a recoverable item is created in the dumpster root folder. Performance decreases when many unnecessary recoverable items are in the folder.

Resolution

To resolve this issue, install the following update rollup:

2529939 Description of Update Rollup 3 for Exchange Server 2010 Service Pack 1

Workaround

To work around this issue, use one or more of the following methods:

1. Disable the IMAP4 protocol.

2. Change to another access method such as Exchange Web Services (EWS).

3. Disable the calendar logging feature when you run the
   Set-Mailbox -Identity <user-identity> -CalendarVersionStoreDisabled:$true command.

Status

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.