Article ID: 2768656 - View products that this article applies to.
Expand all | Collapse all


If you have lots of items in any single folder, you may experience performance issues during certain operations in Microsoft Outlook when you use Cached Exchange Mode or when you use an Outlook data (.pst) file. These performance issues are especially noticeable when you switch into and out of that folder. Additionally, you may notice decreased performance in Outlook if the Inbox, Calendar, Tasks, Sent Items, and Deleted Items folders contain lots of items.


This problem can occur if you have more than 50,000 items in a folder.


When you have folders that have 50,000 or more items in an Outlook data (.pst) file or an offline Outlook data (.ost) file, move items from the larger folders to separate or smaller folders in the same mailbox or data file. Optionally, if you have a Microsoft Exchange online archive, you can move items to it.

You can use the Office Configuration Analyzer Tool (OffCAT) to determine which folder(s) are near or above this item limit. You can click the following link to download and install the OffCAT tool.

OffCAT download

More information

Outlook only uses an .ost file if the Exchange email account is configured to use Cached Exchange Mode. When Outlook is configured to connect to the Exchange mailbox in online mode, an .ost file is not used. If your Outlook client is connected to Exchange in online mode, and you do not have high item-count folders in a .pst file, the performance issue may be on the server.

The following articles list Exchange server performance issues that can occur when a mailbox has high item counts and when restricted views are being used. Additionally, the articles explain how the overall client user experience can be affected by these performance issues.
  • Exchange Server 2010
    Understanding Database and Log Performance Factors
  • Exchange Server 2007
    Understanding the Performance Impact of High Item Counts and Restricted Views
  • Exchange Server 2003
    905803 Outlook users experience poor performance when they work with a folder that contains many items on a server that is running Exchange Server


Article ID: 2768656 - Last Review: September 18, 2013 - Revision: 5.0
Applies to
  • Microsoft Outlook 2013
  • Microsoft Outlook 2010
  • Microsoft Office Outlook 2007

Give Feedback


Contact us for more help

Contact us for more help
Connect with Answer Desk for expert help.
Get more support from