How to Fix “The File username.ost is in use and cannot be accessed” error in Outlook?

When you receive the error “the file username.ost is in use and cannot be accessed,” it means that the OST file you’re trying to open is already in use by some other application. But, this is not the only reason behind this error, there are some other issues as well, such as;

  • Lack of synchronization between the OST file and Exchange mailbox
  • Weak internet connection
  • Unauthorized attempts to access the same OST file
  • Conflict of other programs with Outlook application

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