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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.
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.
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