Outlook: Solve “The file username.ost is in use and cannot be accessed” Error

Microsoft Outlook 2016 crashed on me today. When I tried to launch it again, I received an error that said:

Outlook Data File – The file C:\Users\user name\AppData\Local\Microsoft\user name\username.ost is in use and cannot be accessed. Close any application that is using this file, and then try again. You might need to restart your computer.

 Restarting my computer was not an option at the time. Here’s how you can solve the issue.

Fix 1 – Kill All Outlook Related Processes

1. Right-click the task bar and select “Task Manager“, or press CTRL + ALT + Delete, then choose “Task Manager“.
2. Select the “Processes” tab.
3. Look for anything named “Communicator“, “Lync“, “ucmapi” or “Outlook“. If you find any items related to these, select it, then select the “End task” button.

Fix 2 – Restart Computer

Rather than spend all day trying to figure out the process that is hogging the file, try just doing a simple computer restart. Select “Start” > “Restart“.

Fix 3 – Disable Cached Exchange Mode

1. Close Outlook.
2. Open “Control Panel” and select “Mail“.
3. Select the “Email Accounts” button.
4. Choose the “Email” tab.
5. Highlight the email address giving you the warning (email.ost), then select “Change“.
6. Uncheck “Use Cached Exchanged Mode“.
7. Select “Next” then “Finish“.

Now attempt to launch Outlook again. Hopefully, it works fine now. This is more of a workaround than a fix, as it switches to where you are using Outlook direct from the server. This may cause speed issues. If you have problems with Cached Exchanged Mode disabled, try enabling it again at a later time by rechecking the “Use Cached Exchanged Mode” box in step 6 above.

If neither of these fixes work, you might have another computer or program grabbing the OST file. Makes sure anything that uses Outlook is completely closed on any workstation that may interface with Outlook.

Thanks to Alix Jimenez for Fix 3.