How do I manage and reduce Exchange mailbox sizes?

Your Exchange quota includes all your email, attachments, calendaring data, task items, and more. We strongly recommend that you manage your quota by deleting items you do not need and clear out the deleted items weekly. Your ACCC Exchange mailbox grows as you create and receive items. When you delete items, the size of the Outlook Data File (.pst and .ost) file might not decrease in proportion to the data that you deleted. There are several ways to manage your mailbox and reduce the size of the data file.

Checking mailbox usage

  • Outlook Web App - Log in to Outlook Web App (OWA) then hover your mouse over the top folder of your mailbox, displaying your name. A graph will display your quota usage and will contain a caption letting you know how much space you are using.
  • Outlook 2010 / 2013 - Click on the File tab, click Info, then your quota usage info will appear in the Mailbox Cleanup section.
  • Outlook 2011 for Mac - Click Outlook then Preferences then General in the top menu bar. Uncheck the box Hide On My Computer folders. Right-click on the gray MIT header in the Mail Folders list. This counts the usage on all folders. Click Folder Properties then, click on the Storage tab.
  • Entourage 2008 - Control click on the Exchange Icon, then Select Folder properties, then Select Storage.
  • Outlook 2007 - Go to the Tools menu, then Mailbox Cleanup, click View Mailbox Size, then click on Server Data to find out your quota usage on the email server.

Consequences of a large Outlook/Exchange mailbox size

Not only do you have a fixed amount of space for your mailbox, but a large amount of items in your mailbox can reduce the efficiency of your Outlook-Exchange connection. Individual folders can experience access problems with large amounts of emails stored. Keeping old email messages stored on your local computer can help speed up your Outlook-Exchange connection.

Using personal folders

The best way to keep your Exchange mailbox size to a manageable level is to move non-critical email out of your Exchange Mailbox and into your Personal Folders. While email residing in your mailbox is stored on the server, email that are saved on your Personal Folders are kept on your computer. To create your Personal Folders, follow these simple steps:

  1. From your Outlook session, click on the menu Tools and select Email Accounts.
  2. Under Email click on View or change existing email accounts and click Next.
  3. Click on New Outlook Data File and choose Office Outlook Personal Folders File (.pst) and click OK.
  4. Select the file name and destination and click the OK button.
  5. From the Create Microsoft Personal Folders dialogue window, you can select the Name to appear in your outlook session.
  6. Choose No Encryption.
  7. If you choose to set a password, keep in mind that there is no way to reset this password, then click OK to finish.

You are now ready to sort and file away emails in this newly created Personal Folder which is visible in the Folder List.

Setting up Auto-Archive in Outlook to reduce mailbox size

In order to automatically reduce the size of your mailbox, you can active an Auto-Archive feature to handle this for you. This feature automatically downloads and saves emails onto your local hard drive into its own Archive folder.

  1. Within Outlook, go to Tools, Options, the Other tab and click on AutoArchive button.
  2. From this menu, you can set the preferences for which you want to base your Auto-Archive on.
  3. Select the Box to Run AutoArchive every __ days and select a number of days you feel comfortable with. 7 or 14 are recommended choices.
  4. Under Default folder settings for archiving select either 3 or 6 months.
  5. You can click on Apply to enable these settings to all folders now, unless you know very well what you're using and would like to configure each folder individually (recommended for those who would like to fine tune further), then click OK.
  6. You may then need to Activate Auto-Archive on individual folders.
    1. Right-Click the Folder you wish to Auto-Archive and select Properties from the menu
    2. Select the AutoArchive tab in the folder Properties window.
    3. Select to either 'Archive items in this folder using the default settings' or 'Archive this folder using these settings:' and then configure specific settings for that folder.
    4. Click Apply. Then Click OK to close the window.
    5. This folder will be auto-archived the next time the process runs.

You can also fine-tune specific folders within your mailbox. This can be important if there are some folders that you want to maintain emails for a longer period.

  1. Right clicking on each folder under the 'Mailbox - your name' tree, clicking Properties and the AutoArchive tab gives you more fine-tuning power.
  2. By default, the choice 'Archive items in this folder using the default settings' would be chosen. It is the settings we just configured from the previous section.
  3. Choose 'Archive this folder using these settings:' if you want this folder to differ from the rest. Both retention and location can be changed individually.

Removing attachments from an email and keeping the email

Sometimes you will be sent an email with very large attachments. While there may be plenty of space on your hard drive (or server) the attachment can cause your Exchange Mailbox to grow in size very quickly, but there's an easy solution.

  1. Right click the attachment that you want to save and choose 'Save As'.
  2. If there are multiple attachments, go to File, Save Attachments,Save All Attachments to save them all to your hard drive.
  3. Once saved to your hard drive, right click the attachment again and select 'Remove'.
  4. The file is removed from your email, but is saved to your hard drive.

Manage your mailbox

The Mailbox Cleanup tool is used to view the size of your mailbox and individual folders. You can also archive older items to an archive (Outlook Data File .pst) and empty your Deleted Items folder.

Start by clicking the File tab followed by Cleanup Tools and then click Mailbox Cleanup and do any of the following:

  • To view the total size of your mailbox and any individual folders it contains, click View Mailbox Size.
  • To locate older items, select Find items older than n days, enter a number between 1 and 999, and then click Find.
  • To locate large items, select Find items larger than n kilobytes, enter a number between 1 and 9999, and then click Find.
  • To move older items in your mailbox to an archive Outlook Data File (.pst), click AutoArchive.

Outlook Data Files (.pst) are saved on your computer in the Documents\Outlook Files folder. If you are using Windows XP, .pst files are saved in the My Documents\Outlook Files folder.

  • To view the size of your Deleted Items folder, click View Deleted Items Size.
  • To empty your Deleted Items folder, click Empty.
  • To view the size of your Conflicts folder, click View Conflicts Size.
  • To delete the contents of your Conflicts folder, click Delete.

Reduce the size of an Outlook data file (.pst)

If you delete items from an Outlook Data File (.pst), the file is automatically compacted in the background when you’re not using your computer and Outlook is running. You can also manually start the compact command immediately. It might take several minutes to compact an Outlook Data File (.pst).

  1. Delete any items that you do not want to keep.
  2. Open the Folder List view. At the bottom of the Navigation Pane, click the Folder List icon or press CTRL+6.
  3. In the Navigation Pane, right-click the Deleted Items folder, and then click Empty Folder.
  4. Click the File tab.
  5. Click Account Settings, and then click Account Settings.
  6. On the Data Files tab, click the data file that you want to compact, and then click Settings, and click Compact Now.

Note: You do not have to exit Outlook after you compact an Outlook Data File (.pst) file.

Reduce the size of an offline Outlook data file (.ost)

An offline Outlook Data File (.ost) is exclusive to Microsoft Exchange Server accounts. Reducing the size of an offline Outlook Data File (.ost) might improve performance if you have deleted many items. This procedure does not delete any items saved on the computer running Microsoft Exchange Server.

  1. Delete any items that you do not want to keep.
  2. Open the Folder List view. At the bottom of the Navigation Pane, click the Folder List icon or press CTRL+6.
  3. In the Navigation Pane, right-click the Deleted Items folder, and then click Empty Folder.
  4. Click the File tab, then click Account Settings, and then click Account Settings.
  5. On the Data Files tab, click the data file that you want to compact, and then click Settings, then click Compact Now.


  • To delete an item from both the online and offline folders, select the item, and then press DELETE.
  • If you are using an Exchange account in Cached Exchange Mode, only the selected folder will be cleared from the offline Outlook Data File (.ost). The items on the server will be downloaded again to the .ost file on the next synchronization, if the folder is selected under Folder Options in Send/Receive Settings for the Exchange Server e-mail account.
  • When you use Cached Exchange Mode, public folders favorites are not synchronized by default. If you choose to synchronize public folders, the size of your offline Outlook Data File (.ost) might increase significantly.

Need help?

Last updated: 

March 12, 2013

Browse by tag