This article will help you diagnose and resolve issues where users of Microsoft Office Outlook experiences poor performance when they work with Microsoft Exchange Server 2007/2010/2013/2016.

When this issue occurs, Outlook users may receive one of the following messages:

  • Requesting data from Microsoft Exchange Server.
  • Outlook is retrieving data from the Microsoft Exchange Server.
  • Trying to connect.
  • Not Responding.

Here are the recommended item counts from Microsoft:

  • Exchange 2007 No more than 50000 messages in a single folder
  • Exchange 2010/2013/2016 No more than 100000 messages in a single folder.

In cached mode we are storing a database in the form of a .OST file on your computer.  This OST contains a copy of the mailbox but it also has to work with this database locally for any requests. Even though the recommended number of items for newer versions of Exchange is quite high, having a large number of items in a folder means that operations in that folder will take longer, because most of the low-end computer hard drives are delivering less than ¼ of the performance Intermedia Exchange servers can provide.

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, the number of users, and the number of restricted views. We recommend that you maintain no more than 5,000 items in a folder. Below you will find some ways to do that as well as some other ways to improve Outlook performance.

  1. Archive your data Read the Knowledge Base article on Overview Of Data Archiving Options for information on various archiving options, both client-side and server-side. 
  2. Outlook 2013/2016 - Mail to keep offline Outlook 2013/2016 users may choose how much data they would like to store in their OST file. For example, you can set it to keep 6 months worth of mail offline. It doesn't mean all the other data is inaccessible, it just means that all the other data is accessed in the Online mode. It doesn't affect any Outlook functionality, e.g. when searching, you are still searching through your whole mailbox. mail to keep offline
  3. Repair your OST file often If you have a large OST file, it would be a good idea to repair it often. Read the Knowledge Base article on How To Use the Inbox Repair Tool (scanpst.exe)
  4. Disable unnecessary add-ins Review the list of add-ins that are running in Outlook and disable the unnecessary 3rd-party ones, like iTunes, antivirus add-ins, Business Contact Manager or ACT!.
    • Outlook 2010/2013/2016: File > Options > Add-ins
    • Outlook 2007: Tools > Trust Center > Add-ins
  5. Move folders and multiple items in OWA Moving big folders or a lot of items at once in desktop Outlook client takes significant time to synchronize with the server. Moving a lot of items can often cause disruption in the mailbox if the connection fails for a moment, for example, due to high network usage or your computer’s CPU load. On the other hand, changes made in Outlook Web App occur directly on the server and thus happen instantaneously.