What is OST file?

OST file is known as offline storage table. It is the local copy of your Exchange mailbox. It contains your all mailbox items including emails, folders, posts, appointments, meeting requests, contacts, distribution lists, tasks, task requests, journals and notes etc.

While working offline, you can send emails which are actually stored in the offline Outbox, you can also receive new messages from the other online mailboxes.To reflect these changes to your mailbox on the Exchange server you need to synchronize the offline folders with the Exchange server. MS Outlook provides you an option to synchronize only a specific folder, a group of folders, or all the folders.

The Default location of OST file is:

• For Windows 95, 98 and ME, the folder is:

C:\Windows\Application Data\Microsoft\Outlook

or

C:\Windows\Profiles\user name\Local Settings\Application Data\Microsoft\Outlook

• For Windows NT, 2000, XP and 2003 server, the folder is:

C:\Documents and Settings\user name\Local Settings\Application Data\Microsoft\Outlook

or

C:\Documents and Settings\user name\Application Data\Microsoft\Outlook

• For Windows XP, the folder is:

C:\Users\user name\AppData\Local\Microsoft\Outlook

You can change the default path of your OST file by using following steps:

If you want to set a different location for ost-files, then you’ll need to add another registry key;

Key: HKEY_CURRENT_USER\Software\Microsoft\Office\<version>\Outlook

Value name: ForceOSTPath

Value type: REG_EXPAND_SZ

Value: path to your storage folder

Size limit for MS Outlook OST file:

Microsoft has set a default size limit for MS Outlook OST files as 2 GB for MS Outlook 2000 and 2002. When the Outlook .ost file exceeds this pre-defined size limit, it may lead to corruption. OST file corruption makes it difficult to recover user data from it as it becomes inaccessible. A corrupt OST file will fail to synchronize with the Exchange Server leading to cardinal data loss.

How to configure OST file size limit?

You can configure OST file size limit as required using Outlook registries. MS Office Outlook versions 2010, 2007 and 2003 support American National Standards Institute (ANSI) and UNICODE personal folders (.pst) and offline folder (.ost) files. So you can configure size limit for both OST and PST files. The MaxFileSize registry entry will help you in doing so.

MaxFileSize registry entry - It determines the absolute maximum size that OST file can grow to. After this maximum size is reached, Outlook does not permit the size of the file to grow beyond this size.

Thus, OST file is a very important file in MS Outlook as it lets us  perform offline updates to the exchange server mailbox. These changes are reflected to the exchange server mailbox when synchronization with the exchange server is done.If the data on Exchange server gets corrupted or the server crashes, still you can retrieve your data from the offline folder file on the local computer that contains a copy of Exchange mailbox.