Reduce Mailbox Size

If your mailbox gets to large there are a series of fixes listed below that can lower the size of the mailbox without limiting your access to mail.

There are a few alternative strategies you can employ to manage the OST file size effectively:

• Adjust Sync Settings for Shared Mailboxes: Continue reducing the amount of mail to sync for shared mailboxes. You can customize this setting for each shared mailbox rather than applying a blanket setting for all accounts.
• Go to File > Account Settings > Account Settings.
• Select the shared mailbox and click Change.
• Adjust the Mail to keep offline slider to a shorter period.
• Use Online Mode for Shared Mailboxes: Instead of caching shared mailboxes locally, you can configure them to be accessed in Online Mode. This will prevent the shared mailbox content from being downloaded into the OST file.
• Go to File > Account Settings > Account Settings.
• Select the shared mailbox and click Change.
• Uncheck the "Use Cached Exchange Mode" option.
• Disable Caching for Shared Mailboxes: One of the most effective solutions is to disable the caching of shared mail folders. This will prevent shared mailbox data from being downloaded to the local OST file, thus keeping its size manageable. Here’s how to do that:
  1. Open Outlook.
  2. Go to File > Account Settings > Account Settings.
  3. Select your account and click on Change.
  4. Click on More Settings > Advanced tab.
  5. Uncheck Download shared folders.
  6. Click OK, then Next, and finally Finish to apply the changes.

This setting ensures that only the primary mailbox is cached, while shared mailboxes remain online, which may slightly increase network load but significantly reduce the size of the OST file.

Add Shared Mailboxes as Separate Accounts: Another effective method is to add shared mailboxes as separate accounts. This approach creates individual OST files for each shared mailbox, which keeps the size of each file below the limit. To do this:

1. Open Outlook and go to File > Account Settings > Account Settings.
2. Click on New to add a new account.
3. Enter the shared mailbox's email address and follow the prompts to set it up.
4. Ensure that the account is set up as a separate profile to maintain its own OST file.

This approach allows users to switch between mailboxes without the performance lag caused by a single large OST file

• Adjust OST File Size Limit via Registry: If necessary, you can increase the OST file size limit by modifying the Windows Registry. However, this is generally not recommended as it can lead to instability. If you choose this route, ensure you back up the registry first. Here are the steps:
  1. Press Win + R, type regedit, and hit Enter.
  2. Navigate to:text
     HKEY_CURRENT_USER\Software\Microsoft\Office<xx.0>\Outlook\PST Replace <xx.0> with your version of Office (e.g., 16.0 for Office 2016).
  3. Right-click and create a new DWORD (32-bit) Value named MaxFileSize and set it to a higher value (in MB).
  4. Create another DWORD (32-bit) Value named WarnFileSize to set a warning threshold.

Note: This method should be used cautiously, as it may lead to performance issues if the OST file becomes too large.

1. Enable AutoArchive: Enable AutoArchive to periodically move older items to an archive PST file. This will help keep the OST file size under control while still providing access to older emails.
   • Go to File > Options > Advanced.
   • Click AutoArchive Settings.
   • Configure the settings to move items older than a certain period to an archive file.
2. Move Shared Mailboxes to Online Archive: If your organization uses Exchange Online Archiving, you can move older items from shared mailboxes to an online archive mailbox. This helps to keep the OST file smaller by offloading older emails to the archive.
   • Access the Exchange Admin Center (EAC).
   • Navigate to Recipients > Shared.
   • Select the shared mailbox and enable the Archive feature.

Steps:

1. Open Outlook.
2. Navigate to File > Account Settings > Account Settings.
3. Select the shared mailbox and click Change.
4. Uncheck "Use Cached Exchange Mode" or adjust the "Mail to keep offline" slider.
5. For AutoArchive, go to File > Options > Advanced > AutoArchive Settings.
6. For Exchange Online Archiving, access the EAC and enable the Archive feature.

Regular Maintenance and Cleanup

Encourage users to regularly clean up their mailboxes by:

• Using the Mailbox Cleanup tool to delete old or large emails.
• Emptying the Deleted Items folder frequently.
• Archiving older emails to a PST file.

These actions can help maintain a smaller mailbox size and improve performance.

Vincent Kruggel

July 30, 2025

ArgusIT KB# 130007