Offline Storage Table (OST) file is the data file of Outlook®. It gets created when Outlook profile is configured with MS Exchange Server with Cached Mode enabled.
OST file is the local copy of the mailbox folders created on Server and allows accessing the data when disconnected with the Server. Any changes done to these folders at client or server will be updated at both the ends with there is connection between both of them. This process is called Synchronization. When an OST file is generated, it offers a set of special folders to save mailbox data like Inbox, Outbox, Sent Items, Drafts, Contacts, Calendars, Tasks, Notes, and Journals.
OST File and Technical Insights:
MAPI clients (like Outlook) uses an OST file to access items without connecting with the Server. The folders of OST file are the slave replica of the mailbox folders saved on the Server. An OST automatically encrypts its contents and thus an authentication is required from the clients' end to access the data stored in it.
When Outlook is configured with Exchange Server, an encryption key is generated using MAPIEntryID GUID for the mailbox. This key is then saved in the mailbox on Server and in the MAPI profile in the System's registry.
Whenever the client tries to access the OST file data, the encryption key at both the places is checked. If it exists and match, the access to OST file contents is provided to the user. If the encryption key differs due to any reason, say new Outlook profile, change in mailbox settings, etc. you cannot open the OST file.
Since Exchange Server is deployed by organizations to have a private messaging environment, the encryption of OST file proves to be a valuable asset. In most of the cases, users cannot read OST file because of attempts to open OST in different profile. If the mailbox is deleted from the Server or recreated, the it results in master ID change and then even if the alias name, distinguished name, or account name is the same, the OST file reverts back with an error and refuses to open.
Point to be considers when you are working with OST
- An OST File can be opened in the MAPI profile with which it is originated.
- Automatic synchronization of OST folders with mailbox on Server. However, special synchronization has to be set up for the public folder replicas and folders created by the user.
- An OST file is encrypted by default. It does not require password protection due to the relation with mailbox on server.
- Two OST files cannot be opened at a time in Outlook.
Outlook 2013 Welcomes New Protocol for OST:
With Outlook 2013 edition, Microsoft has changed the way how IMAP accounts are handled. With IMAP access of a mail account for Outlook 2013, an OST file gets created instead of PST.
This feature is available from Outlook 2013 only and the resultant OST will share the same attributes as the OST file with Exchange Server mailbox.
Location of OST File:
Location of OST file can vary depending upon the version of Windows and Outlook used. Below mentioned is the default location of an OST file on the machine.
||Location of OST
|C:\Documents and Setings\%username%\Local Settings\Application Data\Microsoft\Outlook
||C:\Documents and Settings\%username%\My Documents\Outlook File\
OST File and Related Problems:
Problem 1: Orphaned OST File
Exchange Server mailbox and OST file shares Parent-Child relationship which is protected with the MAPI encryption key. Therefore, if the child is disconnected from the parent, it will result lose its identity. Similarly, if an OST file loses connection with Exchange Server mailbox, it will become Orphan (just like a child becomes without its parents). An orphaned OST file is inaccessible.
Some of the situations where an OST file can become orphaned are mailbox deletion, Outlook profile recreation, changes in the alias name of the mailbox, the mailbox transferred to new database, etc. In these cases, the master ID will get changed and OST will refuse to load.
Problem 2: Corrupt OST File
Just like any other file saved on the local machine, an OST file can become the victim of hardware and software related issues and get corrupt. This includes improper working of system software like OS, hardware related problems like bad sectors of hard drive, software applications like file level scanners un-aware of OST structure. All such issues cause OST corrupt and consequently inaccessible.
Problem 3: Outlook Configuration Errors
For smooth functioning of OST file, it is important that is it properly configured with Exchange Server. In absence of proper configuration, errors will be prompted on screen that affects normal functioning of OST file. Also, error messages can be the result of corruption or synchronization issues.
Troubleshooting OST File:
To work around issues related to OST file, there are some tricks that can prove to be of great help. Few of them are mentioned below:
1: OST Integrity Checker
Scaost.exe or OST Integrity Checker is a free utility from Microsoft that checks for synchronization issues between Exchange Server and OST file and then fixes them. This tool scans the OST file and corresponding Exchange store, tests for any issues between them and tries to fix it. To use this tool, it is necessary to be connected to Exchange Server.
2: OST Archiving to PST
Outlook data files fall into the trouble zone due to the large size. To avoid the consequences of large sized OST file, it is better to archive OST emails into the PST file format. This can be done using the auto-archiving feature of Outlook.
Note: By default, an OST file is created in Outlook when configured with Exchange Server but a PST file can be created and OST data can be archived into it.
3: Third Party Application
In the above section, we shared adverse states of OST file: orphan and corrupt. In that case the only solution to get access over the data saved in OST file is to recover an OST file into PST. These third party tools help in retrieving data from OST file that is orphaned or corrupt.
An Outlook OST file offers a solution to securely save the mailbox data that is stored on Server database. While its rigid security model has some advantages at the enterprise level, it also has some disadvantages. To deal with the problem, there are solutions that can be adopted for efficient working.