Tips for Improving Performance of Microsoft 365 Outlook

This article addresses some issues that affect the overall performance of Outlook. Symptoms of less than optimum performance of Outlook can be:

• emails not sending

• sent emails being delayed

• attachments not downloading

Included in this article are things to check and/or adjust that may help to provide better performance of Outlook in your desktop app:

Mailbox Size

You may experience decreased performance of Outlook when the Inbox, Calendar, Tasks, Sent Items, and/or Deleted Items folders contain too many items. (Deleted Items are currently kept indefinitely and are not deleted after a certain amount of time.)

How to check the size of your mailbox folders

1. Login to OWA

2. Click on the Settings gear in the top right of your screen and choose View all Outlook settings

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3. In the left-hand menu, choose General and then Storage

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Moving Items Out of Default Folders

The default email folders in Outlook are Inbox, Drafts, Sent Items, Deleted Items, and Junk email. If your default folders contain a lot of items that you would like to save, moving them into new folders can improve Outlook’s performance. Name your folders in such a way that their contents can be identified, i.e. “Inbox2020” if you are moving emails from 2020, or “Old Sent” if you are moving all sent emails not from the current year, etc.

Items moved into new folders will still be located when searching a mailbox.

1. Right-click the folder from which the messages will be moved (i.e. Inbox, Sent, or Deleted)

2. Choose New Folder. An untitled folder will open below the parent folder :
   
   

    

3. Provide a name for the folder and hit enter

4. Filter your messages to easily organize the messages. Go back to the folder where the items will be moved from. In the window where your email messages are listed, click on the By Date at the top right-hand side to activate the filter and choose how to Arrange by or Sort to arrange your mail to put the relevant items at the top (for example Date > Oldest to Newest if you’re moving the oldest emails). Note: if cached mode is on, you can only view emails within a set time range. See the Cached Mode section below for more information.
   
   

    

    

5. Select the group of items you wish to move by using Shift + Click to select the group.

6. Right-click the selection and choose Move To > (newfoldername)

Tip 1: Move To is more accurate than dragging and dropping. Use this to avoid accidentally dropping files into an unintended folder

Tip 2: If you are moving a large number of files, it may be better to do the move in batches.

Outlook Connection Status

In this window you can check the status, the number of failed requests (under Req/Fail), and which mailbox is experiencing the issue.

1. Open the System tray

2. Ctrl + right click on Outlook

3. Choose Connection Status …

4. Under Status it should say Established

    

5. Req/Fail lists the number of requests that are made during the session and the number of those requests that failed for the specific connection. Any mailboxes with a high fail rate may be causing issues

Viewing and Removing Add-ins

1. Go to File > Manage Add-ins

2. Scroll down to Manage Add-ins

This will take you to the following page:

Slow or Disabled Add-ins

Review the slow or disabled add-ins by going to File > Slow and Disabled COM Add-ins

Slow add-ins that are no longer being used should be removed. If it is required, check if there are other Outlook settings that could optimize its performance in Outlook.

Cached Mode

Cached mode saves a copy of the mailbox to the computer.

Note: currently the Outlook for Mac desktop versions only support cached exchange mode.

This setting can be found by going to File > Account Settings > Account Name and Sync Settings

The slider can be used to choose the time frame of emails to be downloaded. Uncheck the box beside “Use Cached Exchange mode to download email to an Outlook data file” to turn off cached mode.

Number of Mailboxes

Having too many shared mailboxes and folders can cause slow performance in Outlook. The limit will depend on the size of the mailboxes and folders. It is recommended to remove any mailboxes that don’t need to be open. These can still be accessed in OWA.

If the mailboxes open automatically after removing them and restarting Outlook, they may be auto mapped. This setting can be changed by an Exchange admin by submitting a request via Jira and including the usernames of the client and the usernames of the mailboxes that need to be removed.

Microsoft Reference Articles

Application pauses if having large data file - Outlook | Microsoft Docs

Slow performance if many shared folders or mailboxes open - Outlook | Microsoft Docs

Performance issues for too many items or folders - Outlook | Microsoft Docs

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