Shared Mailbox Accounts and Outlook's Offline Cache

Shared Mailbox Accounts can affect the offline cache file (OST) of Windows Outlook clients. When someone “owns” a large Shared Mailbox account (e.g., having many large attachments), or has extensive privileges to a Shared Mailbox account, the local online cache (OST file) of their Outlook client becomes too large. The impact on the offline cache file depends on the following:

• Client version: Outlook cache size slider also controls content from shared mailbox accounts. 

• How much is being shared: A full access delegate (i.e., an “owner”) will automatically have access to the entire contents of a shared mailbox account, whereas a user with share permission will be limited to the content of what folders have been shared.

Solutions

The simplest solution is to turn off "Download shared folders" using the steps below. If you do this, Shared Folders will no longer be stored in your offline cache. As a result, you’ll be reading these folders directly from the Exchange Server, so you will experience delays as the mail is being accessed and downloaded.

1. In the Outlook client, right-click over your account name and select Data File Properties. This takes you to an Outlook Today Properties window.

2. In the General tab, click the Advanced button.

3. In the Microsoft Exchange window, select the Advanced tab.

4. In the Cached Exchange Mode Settings, remove the check mark from Download shared folders.

5. Restart Outlook. 

Additional options for Outlook:

Decrease the amount of mail to keep offline using the steps below. The “Mail to keep offline” slider in Office also affects shared folders. The default setting is 12 months. You can set the slider to cache a shorter period of time and thus reduce the size of your offline cache.

1. Click the File tab in Outlook.

2. Under Account Information, click Account Settings and select Account Settings.

3. In the Account Settings window, select the Email tab and make sure your Microsoft Exchange Account is selected. Click the Change icon.

4. In the Change Account window, move the Mail to keep offline slider to the right. Try reducing the current amount by half.

5. Click Next and Finish.

6. Restart Outlook.