Fixed: Outlook is using an old copy of your Outlook data file (.ost)

Today I’ve found the solution to Outlook’s most common error which anonymously shows a prompt on the screen when you restart your computer and open Outlook account or restart MS Outlook. 

 

 If Outlook displays the error message such as "Outlook is using an old copy of your Outlook data file (.ost). Exit Outlook, delete the file, and restart Outlook. A new file will be automatically created the next time you initiate a send/receive," when you open or restart Outlook email client, probably the OST couldn’t be properly accessed by Outlook. 

The error indicates that Outlook can’t access the OST file because Outlook or Windows has crashed. However, it is a cached file: the original one of your mailbox is still safe on your Exchange server; so it is safe to delete the OST file. Also, the next time your Outlook profile will connect to Exchange server, a new OST file will be created at the same location. The process to search and delete the OST file is very simple.

If you’re using Windows 7:

1.       Open Computer from the Start menu.

2.       Navigate through File > Organize > Folder and Search Options, and click the View tab.

3.       Click 'Show hidden files, folders and drives' and uncheck the 'Hide extensions for known file types' option. Click OK

If you’re using Windows 10:

1.       Open File Explore and click View menu. Check File name extension option and the Hidden items.

 

 

 

1.       Navigate to the below location (in the Computer (Win 7) or File Explore (Win 10)) -

    C:\Users\<username>\AppData\Local\Microsoft\Outlook

Now look for the associated .ost extension file and delete it (Make sure Outlook is closed). Next, open Outlook once again, and try to connect to the server. Outlook will take some extra time to recreate the cache copy because you have recently deleted the cached OST file. However, you can notice the 'updating folder' notification in the lower side of the window. Once all the cache folders are updated, your Outlook will connect to the server without any error.

However, if the issue persists and couldn’t be resolved by deleting the OST file, you can follow the below method -

1.       Open Control Panel and navigate to Mail -> Show Profiles, and select your Outlook profile.

2.       Once the file is selected, navigate to Properties -> E-mails Accounts

3.       Now change the existed email accounts or select View and click Next. Select Microsoft Exchange Server account and click Change.

4.       A new dialog box will prompt on your screen which displays the mailbox server and your username, click More Settings option.

5.       Click Security tab in the newly opened window and click to select the Encrypt data between Microsoft Exchange Server and Microsoft Office Outlook check box, and click OK.

Now, you will no longer receive the error message once the process is completed.

However, in some critical scenario, if the both solution didn’t work for you, it’s time to disable the DFS and disable cache Exchange mode.

Hopefully, the next time you reboot MS Outlook, no such error message will appear on your screen.