.ost and .pst files

Bottom Line: Never delete .pst files, as they are the only local copy of your emails. You can delete .ost files to fix Outlook glitches, but you must close Outlook first and let it resync afterwards.

What are .ost and .pst files?

If you use the Microsoft Outlook desktop app for your email, your computer stores all of your messages, attachments, contacts, and calendar events inside massive data files ending in either .ost or .pst.

• .ost (Offline Storage Table): This is just a cached copy of what already exists on your email server (like Office 365 or Exchange). It allows you to read emails without an internet connection.
• .pst (Personal Storage Table): This is a hard, local backup of your emails. It is often used for POP3 email accounts or old archives. If it’s in a .pst file, it might not exist anywhere else in the world.

Can you delete them?

⚠️ Use Caution. It depends entirely on the file extension.

For .pst files: ❌ Danger. Do not delete these. If you delete a .pst file, those emails are gone forever.

For .ost files: ✅ Safe. Because .ost files are just a mirror of the cloud, they are perfectly safe to delete. In fact, IT professionals delete .ost files all the time to fix corrupted Outlook profiles or search glitches.

How to safely delete an .ost file

If Outlook is running terribly slow or failing to receive new emails, rebuilding the .ost file is the ultimate fix:

1. Close Microsoft Outlook completely.
2. Press Windows Key + R to open the Run dialog.
3. Paste this exact path and press Enter: %localappdata%\Microsoft\Outlook
4. Look for the massive file that ends in .ost (it is usually named after your email address).
5. Delete it (or simply rename it to backup.ost.old just to be safe).

What happens after you delete it?

The next time you open Outlook, it might take a long time to load. It will say “Updating this folder” at the bottom of the screen. Outlook is simply contacting the Microsoft servers and re-downloading a fresh, uncorrupted copy of all your emails to build a brand new .ost file.