Introduction

If you’re using Outlook with Exchange Online, you’ve probably seen the option to “Download all emails to your computer”. It sounds convenient, but syncing your entire mailbox—and shared mailboxes—into an OST file can cause serious performance issues and business risks. Here’s why you should limit OST sync to 12 months or less, and why shared mailboxes make this even more critical.

What’s an OST File?

An OST (Offline Storage Table) file stores a copy of your mailbox on your computer for offline access. It’s useful for working without an internet connection—but syncing everything (including shared mailboxes) is a recipe for trouble.

Why You Should Limit OST Sync to 12 Months or Less

1. Performance Impact

  • Outlook Slows Down: Large OST files (especially when shared mailboxes are included) make Outlook sluggish, increase load times, and cause frequent freezes.
  • Search Problems: Indexing huge OST files takes forever, making searches painfully slow.

2. Computer Resource Drain

  • Storage Hog: Syncing all emails—including shared mailboxes—can consume hundreds of gigabytes of disk space.
  • CPU & RAM Strain: Constant syncing and indexing hammer your system resources, reducing overall PC performance.

3. Shared Mailbox Impact

  • Massive Growth: Shared mailboxes often contain years of emails from multiple users, dramatically inflating OST size.
  • Sync Conflicts: Multiple shared mailboxes syncing locally can cause Outlook crashes and corruption.
  • Business Disruption: If OST corruption occurs, access to shared mailbox data is lost until IT intervenes.

4. Business Impact

  • Lost Productivity: Slow Outlook = frustrated employees and wasted time.
  • Higher Support Costs: IT spends more time troubleshooting performance issues and repairing corrupt OST files.
  • Risk of Corruption: Bigger OST files are more prone to corruption, which can lead to data loss and downtime.

5. Compliance & Security

  • Local Copies Everywhere: The more data stored locally, the higher the risk if a laptop is lost or stolen.

Best Practice

Set your Cached Exchange Mode to 12 months or less for your mailbox and avoid syncing shared mailboxes locally unless absolutely necessary. Older emails remain accessible via Online Archive or server search—so you’re not losing anything.

FAQ

Q: Why not sync everything?
A: Because it slows down Outlook, hogs storage, and increases corruption risk—especially with shared mailboxes.

Q: Can I still access older emails and shared mailbox content?
A: Yes! They’re always available online or in your archive.

Q: What’s the recommended setting?
A: 12 months or less. For most users, 6 months is plenty.

Q: What if I already synced everything?
A: Contact our helpdesk—we’ll help you adjust settings and optimize your mailbox.

Need Help?

📧 [email protected]
📞 321-888-2950
We’ll make sure your Outlook runs smoothly and your data stays safe.