Outlook 2013 OST File

Outlook 2013 introduced many improvements into the most famous e-mail client. Some of these are around Outlook data files (OSTs).

By default, when Outlook 2013 is installed, a new compressed version of the OST is created. I say “compressed” because apparently Outlook compresses several data fields/values in order to reduce the size of the OST file. This new compressed version is, allegedly, up to 40% smaller than OST files created by earlier versions of Outlook.

If you don’t want Outlook 2013 to create a new compressed OST file across the workstations in your environment when you upgrade your users’ Outlook client, use the Outlook Group Policy template (Outlk15.admx) to enable the Do not create new OST file on upgrade policy. You can find this setting under User Configuration\Administrative Templates\Microsoft Outlook 2013\Account Settings\Exchange.

A big change is that when running in Cached Exchange Mode, Outlook 2013 will only cache 12 months’ worth of data from the Exchange server by default! This is part of the new Sync Slider feature, which allows users to limit the number of e-mails that are synchronized locally into the OST. Users will be able to view messages that were removed (or not synced) by scrolling to the end of an e-mail list in a folder and clicking the message Click here to view more on Microsoft Exchange.

Note that this setting does not affect the number of items synchronized in Calendar, Contacts, Tasks, Journal and Notes.

Obviously, users can change how much e-mail to keep offline. To do this:

  1. Start Outlook;
  2. Click Account Settings on the File tab, and then click Account Settings;
  3. In the Account Settings dialog box, double-click your Microsoft Exchange account on the E-mail tab;
  4. In the Change Account dialog box, drag the Mail to keep offline: slider to the desired number of months;
  5. Click Next;
  6. Click OK when advised you have to restart Outlook to complete the configuration change;
  7. Click Finish;
  8. Restart Outlook.

On the other hand, administrators can change this setting on behalf of their users. The cached mode Sync Slider setting is maintained in the Outlook profile settings in the Windows registry. To administer this setting via group policy, you can use the Outlk15.admx and Outlk15.adml group policy templates available from the Microsoft Web site. Outlook uses the following registry data:

  • Key: HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\15.0\Outlook\Cached Mode
  • DWORD: SyncWindowSetting
  • Value: integer value (Decimal) with the following values:
    • 0 = All (entire mailbox)
    • 1 = One month of mailbox items
    • 3 = three months of email items
    • 6 = six months of email items
    • 12 = twelve months of email items
    • 24 = twenty-four months of email items

Another improvement is the Fast Access feature that brings the “instant access” of Online Mode into Cached Mode and improves the robustness of syncing. If, for example, a user was on a slow connection, he/she would typically need to wait a while for the mailbox to be updated to then start working. However, when users start Outlook 2013, they will almost immediately see the most recent e-mails and a completely up-to-date calendar as Outlook prioritizes these while caching the remaining items in the background.

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