Outlook users experience poor performance when they work with a folder that contains many items on a server that is running Exchange Server

Microsoft Office Outlook users experience poor performance when they work with a folder that contains many items on a server that is running Microsoft Exchange Server. Additionally, Outlook users may receive one of the following messages:Note An item is defined as an email message or a folder.

This issue occurs when an Outlook user works with items in a folder that contains many items. Outlook must perform several operations against the Exchange server to retrieve the contents of a folder. Therefore, when there are many items in a folder, additional processing is required to respond to the Outlook requests.

You can help avoid poor performance in Outlook by carefully managing the number of items in folders, especially the Outlook folders that are heavily used. These folders include Inbox, Calendar, Tasks, Sent Items, and Deleted Items, with special emphasis on Inbox, Sent Items, and Deleted Items, because these are the first to be downloaded. This will also pertain to any other folders that are heavily used.

The recommended number of items in a folder depends on several factors. These factors include the client�s proximity to the server, the storage infrastructure, the load on the hard disks (client and server), the number of users, and the number of restricted views.

We recommend that you maintain a range of 2,500 to 5,000 items in a folder, depending on the capacity of the Exchange Server environment. Additionally, you can create more top-level folders or create subfolders under Inbox, Sent Items, and Deleted Items. When you do this, the costs that are associated with index creation are greatly reduced, if the number of items in any one folder does not exceed 5,000.

The following list includes ways that you can help manage the number of items in folders:

• Use folder hierarchies to help keep the number of items in a folder to the recommended values. (We recommend that you use no more than three levels of folders. For example, the Inbox is the top level. Then you have a subfolder labeled "2010" for all email messages that were received in 2010. And then you have a subfolder inside of the 2010 folder named "January" for the January email messages. This folder named January is the third level.)
• Use mailbox manager policies.
• Use client-based archiving solutions.
• Use server-based archiving solutions.
• Use mailbox size limits.

The following list describes why more processing time may be required to retrieve the contents of the folder when there are many items in the folder:

• Folder contents are stored in a table in the information store database. As the number of items increases, there is a corresponding growth in storage complexity. The storage mechanism for the Exchange store is the Extensible Storage Engine (ESE). ESE uses B+ trees data structures to store records. As the number of records increases, the potential number of disk I/O requests that are required to locate the information and traverse the B+ tree also increases. For more information, visit the following Microsoft Web site:
  http://technet.microsoft.com/en-us/library/aa998171.aspx
• As the number of items increases, the chance of the data being physically adjacent on the hard disk is greatly diminished. Therefore, more I/O requests are required than if the data were adjacent on the hard disk.
• Creating restricted views, or views of only a subset of items in a folder, requires more processing from the Exchange server. This behavior occurs because of the overhead that is associated with creating a search folder and populating the search folder with items that match the user's criteria. This process requires each item in the folder to be examined to determine whether each item should be put in the search folder. Therefore, when a folder contains many items, more time is required to create the view. By default, after a search folder is populated, the folder may exist for up to 40 days. For more information about search folders, visit the following Microsoft Web site:
  http://technet.microsoft.com/en-us/library/bb124114.aspx
  Note "Smart folder" in Microsoft Outlook 2011 and "Mail View" in Microsoft Entourage are equivalent terms to "search folder."
• The presence of restricted views affects how quickly modifications can be made to the items in a folder. When a search folder (smart folder for Outlook 2011 and MailView for Entourage) is associated with another folder, more processing occurs when items are added, deleted, or updated in the other folder. This behavior occurs to determine whether the search folder must also be updated. When you establish many search folders, each change must be evaluated against all search folders to determine whether the search folders must also be updated. For more information about search folders, visit the following Microsoft Web site:
  http://technet.microsoft.com/en-us/library/cc535025.aspx
  Note "Smart folder" in Microsoft Outlook 2011 and "Mail View" in Microsoft Entourage are equivalent terms to "search folder."
• When more nonstandard properties are added to a folder view in Outlook, more remote procedure call (RPC) requests may be required to retrieve the property from the Exchange store. When a folder contains many items, more round trips are required to retrieve the data from the Exchange server.
• When you try to view a calendar folder, Outlook must locate all appointments in the specified date range. This requires at least two processing requests. The first request obtains all static appointments in the specified date range. The second request locates any recurring appointments that occur in the specified date range. The time that is required to process the second request is proportional to the number of items in the calendar folder. This behavior occurs because Outlook requests all recurring appointments. After the list of recurring appointments is received, each recurring appointment must be examined to determine whether the appointment occurs in the specified date range. When a folder contains many items, more time is required to process these requests.

Note�If you enable the Outlook option Use Cached Exchange Mode�for an Exchange email account, you may see improved performance.�If you do not use cached mode, Outlook connects to the mailbox in online mode. This results in additional requests to the Exchange server.

For more information on client performance with Exchange Server 2007, see the following article from Microsoft TechNet Web site:

For information on client performance with Exchange Server 2010, see the following article from Microsoft TechNet Web site: