How to Delete an OST File (Outlook)
OST files are the Microsoft Exchange server’s offline replicas of user mailboxes.
This file is located on the computer’s hard drive and is comprised of emails, settings, reminders, etc, and is therefore a local copy of a mailbox that saves changes to Outlook in the absence of internet connectivity. Thus, it is very important for this file to properly synchronize with the Exchange server to reflect any changes made offline.
These files can sometimes become corrupt, which leads to a host of issues, including mailbox inaccessibility or synchronization difficulties. Deleting this file means we can create a new, healthy one, hopefully eradicating pesky problems.
This article will walk you through how to do that.
Note: Make sure the Outlook application is closed before deleting the .ost file, as this cannot be done while the app is running. Where appropriate, it is also a good idea to have a backup of the .ost file to avoid permanent data loss.
Press the Windows key + R to open the “Run” application.
This step can also be performed by clicking on the “search” bar at the bottom left of the screen and typing “Run”.
Copy and paste %LOCALAPPDATA%\Microsoft\Outlook into the Run box and click the “OK” button.
a. You are now in the .ost data file location.
3. Next, locate the .ost file you want to delete.
a. You should see Outlook data files to each mailbox you are connected to via Outlook. Right-click on each one and click “properties”. It will tell you the type of file it is near the top. We only want to delete .ost files, nothing else!
b. Once you have located the .ost file to the specific mailbox you are troubleshooting, right-click and click “delete”.
4. Now reopen the Outlook application. By doing so, the Exchange server automatically creates another .ost file if no synchronization issue exists.
Troubleshooting Tips
To ensure there are no synchronization issues with the Microsoft Exchange server, you can check to see if the user is able to access the mailbox on the browser version of Outlook via http://mail.utoronto.ca.
When the user is logged in on the web browser, navigate to the top-right of the screen to the user profile, click, then click “Open Another Mailbox”.
Type in the mailbox the user is trying to access.
If the user has access to the mailbox, a new window should open with the appropriate mailbox for use.
If the accounts are synchronizing on the web browser, but deleting the .ost file does not fix the errors, try removing the mailbox from the Outlook application and add it back.
Remove the account by following these steps: https://support.microsoft.com/en-us/office/remove-or-delete-an-email-account-from-outlook-1fa900ae-6dc8-468c-b754-10292f7ee47a
Add the account back by following these steps: https://support.microsoft.com/en-us/office/add-an-email-account-to-outlook-6e27792a-9267-4aa4-8bb6-c84ef146101b#PickTab=Outlook_for_PC
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