Resolving OST Corruption
In the worse-case scenario, such a problem with Outlook might indicate that the OST file has become corrupt, leading to the error message informing you that Outlook cannot open default email folders. The OST (Offline Storage Table) file is created by Outlook to handle an Exchange mailbox when your computer is offline. Such a scenario might occur if you are using a laptop, for instance.
If this issue is affecting you, the following message would be displayed:
Cannot open your default email folders. The file filename.ost is not an offline folder file.
The error is usually caused by data corruption, although an inability to open the Exchange mailbox on the computer or an incorrectly configured account can also result in this problem. Using the error message, you should be able to use your Windows search function to find the OST file that is experiencing the problem. There are two things you should do next.
First, make a copy of the OST file, and save this either in the same folder (with the default Copy of… prefix) or elsewhere.
Second, use the ScanOST utility (found in C:\Program Files\Common Files\System\Mapi\1033\scanost.exe by default) for Outlook versions prior to 2010 and check the OST file named in the error for problems. This will resolve the majority of corruption errors.
If fixing the OST file doesn’t resolve the issue, or you are using Outlook 2010, delete the original file (you should make sure you have a copy before doing this) and with your computer connected to the network and able to access the Exchange server (you might check via Outlook Web Access or by pinging the server) restart Outlook; the email application should create a new OST file, and the issue will be resolved.