This article will help you diagnose and resolve issues where users of Microsoft Office Outlook experience poor performance when working with Microsoft Exchange Server.

When such an 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

In some cases it means that you will need to troubleshoot connectivity issues. If you have already checked that webmail works fine and noticed that there is an improvement in Outlook performance in Online Mode, it is likely that your mail client is having difficulties with working in Cached Exchange Mode. Usually, it happens to large mailboxes and, subsequently, large Outlook data files that are stored locally on the machine and are accessed every time Outlook tries to retrieve the mailbox data from the server. The larger this file is, the higher the chances that it might get corrupted or might affect Outlook performance are.

While the overall size of .ost files has a preconfigured limit of 50 gigabytes (GB), it is also possible to increase this limit, however Microsoft recommends avoiding the Outlook data files growing larger than 10 GB. Read the Microsoft KB article on the connection between Outlook data file size and Outlook behavior for more information. If you wish to increase the limit of your .ost file you may also refer to this article.

Important: Outlook is designed to be used as a mail client primarily and it is not designed to be used as a long term archive store of a large amount of email.

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. In particular, Outlook performance is significantly influenced by the number of items in the most commonly used folders such as Calendar, Contacts, Inbox, and Sent Items folder. 

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. 

Here are the Microsoft limitations for the number of items per folder:

  • Exchange 2007: 50000 messages per folder, 500 folders total
  • Exchange 2010/2013/2016/2019: 100000 messages per folder, 500 folders total

Note: due to the nature of shared environment, we do not recommend that you maintain more than 5000 items in a folder.

Below you will find some recommendations on how to improve Outlook performance.

  1. Archive your data

    Read the Knowledge Base article Overview Of Data Archiving Options for more information on various archiving options, both client-side and server-side. 
  2. Outlook 2013/2016/2019 - Mail to keep offline

    Outlook 2013/2016/2019 users can choose how much data they would like to store in their OST file. For example, you can set it to keep only 6 months of mail offline. It doesn't mean that all the other data will be inaccessible, it means that the rest of the data that does not fall in to the chosen period will be retrieved in Online Mode. It does not 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 practice to repair it often. Read the Knowledge Base article on How To Use the Inbox Repair Tool (scanpst.exe) If scanpst.exe didn't help, you can create a new Outlook profile and configure it to connect to your mailbox. We recommend to keep your old profile.
  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, ACT, etc. (check out the full list of plugins that may affect Outlook performance)
    • Outlook 2010/2013/2016/2019: 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 when the connection fails for a moment, for example, because of the high network usage or CPU load of the PC. We highly recommend making such changes via Outlook Web App as in this case all items are moved directly on the server.