When you use Outlook in the Cached Exchange Mode, a copy of the mailbox that is typically stored on the Exchange server is also stored on your local computer. This allows you to quickly access your data regardless of the quality of the connection to the server. Outlook carries out frequent synchronization between the offline folders and the mail server, thereby ensuring that there is complete consistency between the two copies. This way, even if you choose to work offline, all the data will be available to you. This offline data is stored in offline storage tables called OST files. One of the most commonly faced problems with OST files is when the mailbox of a user is deleted from the server and there is no easy way to recover exchange data.

Let us first look at how Outlook actually synchronizes data between the offline folders and the mailbox on the server. When there is a problem with the connection to the Exchange server, Outlook immediately switches to the 'Disconnected' mode or sometimes to the 'Trying to Connect' mode. In this mode, it allows you to continue working on the contents of the offline folders even without a network connection. As soon as the connection is restored, Outlook automatically goes back to the 'Connected' mode and all the changes that were made in the offline folders are copied into the server mailbox. If any new messages had been received in the meantime, these are seen in the offline folders as well. As you can understand, the offline folders or OST files are essentially replicas of the mailbox stored on the Exchange server - if the mailbox on the server is deleted accidentally, it might prove difficult to recover exchange data that has been lost.

Outlook uses the mailbox on the server and the user profile associated with it to synchronize contents with the offline OST files. Sometimes, when the mailbox is deleted from the server your local OST file becomes orphaned and you may find it impossible to access the contents. Microsoft Exchange server has no way of identifying you as the person authorized to access the contents of the mailbox and thus the OST files making it difficult for you to access the OST data as well. Corruption in the OST files is another reason why they may become inaccessible. These factors make it difficult to synchronize the OST files with the mailbox on the server and you have to look at ways in which you can recover exchange data and possibly create a new mailbox.

Wherever possible, Outlook users try to keep copies of their messages as PST files, since these can be accessed locally without requiring any connection to the Exchange server. They can also be backed up making it easy to recover data. If you have not stored your data in PST files, have corruption issues with the backups as well and find that you are dealing with damaged or orphaned OST files, you could use third-party recovery tools such as Advanced Exchange Recovery from DataNumen. They use sophisticated technologies to scan orphaned or corrupted OST files allowing you to recover as many objects as possible from the OST files.