If you are unable to delete emails from a given Outlook folder (not just certain emails), most likely that folder is locked because it is being used by a 3rd party application (for example, it may be scanned by an anti-virus program). This article describes various solutions to this problem. Sometimes, emails can get stuck in your Inbox, Outlook or Sent Items folder and you are unable to move or delete them, while Outlook reports an “unknown error”.