Comprehensive Guide to Managing Large Outlook .OST Files with Shared Mailboxes
When Outlook’s .OST file grows past 50 GB—often due to cached shared mailbox data—you may face slow performance, freezes, or sync failures. This guide covers everything you need to know to shrink your OST file, prevent it from ballooning again, and keep Outlook running smoothly.
1. Limit How Much Mail You Store Locally
Why It Helps
Keeping only recent email cached means your OST file stays lean.
Steps
- In Outlook, go to File > Account Settings > Account Settings.
- Select your Exchange account and click Change.
- Adjust the Mail to keep offline slider to a shorter period (e.g., 3 or 6 months).
- Click Next, then Done, and close Outlook.
- Reopen Outlook. A new, smaller OST file is built with only the selected date range.
2. Stop Caching Shared Mailboxes
Why It Helps
Shared mailboxes often add gigabytes to your OST. Loading them on demand saves disk space.
Steps
- Go to File > Account Settings > Account Settings.
- Select your account and click Change > More Settings.
- Open the Advanced tab.
- Uncheck Download shared folders.
- Click Apply > OK, then Next > Done.
- Restart Outlook to apply changes.
After this, shared mailboxes stay on the server and aren’t stored in your OST.
3. Disable Automapping of Shared Mailboxes
Why It Helps
Prevent unwanted mailboxes from auto-adding and enlarging your OST.
Steps
- Open Exchange Admin Center or connect via PowerShell.
- Run: powershell
Set-MailboxPermission -Identity "SharedMailboxName" ` -User "YourUsername" -Automapping $false - Replace
"SharedMailboxName"and"YourUsername"with actual names. - Restart Outlook and manually add only the mailboxes you need (File > Open & Export > Other User’s Folder).
4. Move Older Mail to an Archive
Why It Helps
Separating old mail into a PST archive keeps your primary OST focused on current work.
Methods
- Archive Mailbox
- In Exchange Admin Center, enable the archive mailbox for large or shared mailboxes.
- AutoArchive in Outlook
- In Outlook, go to File > Options > Advanced > AutoArchive Settings.
- Set a schedule and choose a PST file location.
- Specify age (e.g., 6 months) for moving items.
Archived items live in a separate PST and don’t bloat your main OST.
5. Increase the OST File Size Limit (Last Resort)
Why Use Caution
Raising the limit can degrade performance and risk data corruption if the file grows too large.
Steps
- Press Win + R, type
regedit, and press Enter. - Navigate to:
HKEY_CURRENT_USER\Software\Microsoft\Office\\Outlook\PST
Replace - Create or edit these DWORD values:
- MaxLargeFileSize: Desired max size in MB (e.g., 102400 for 100 GB)
- WarnLargeFileSize: Warning threshold in MB (e.g., 97000 for 95 GB)
- Close Registry Editor and restart Outlook.
Only use this if you can’t reduce OST size by other means.
6. Ongoing Maintenance and Best Practices
- Monitor OST Size
- Check the file size periodically in File Explorer (usually in
%LOCALAPPDATA%\Microsoft\Outlook).
- Check the file size periodically in File Explorer (usually in
- Archive Regularly
- Encourage users to run AutoArchive or move old folders manually.
- Educate Users
- Teach staff how to access shared mailboxes online and subscribe only to necessary folders.
- Set Retention Policies
- In Exchange, configure policies to auto-delete or archive items older than a set period.
- Use Online Mode for Large Mailboxes
- For very large mailboxes, consider using Online Mode instead of Cached Exchange Mode.
By applying these steps—reducing sync periods, disabling shared-folder caching, limiting automapping, archiving older email, and, if needed, raising file limits—you can keep OST files under control, ensuring Outlook remains fast, stable, and reliable.
