Outlook 2010 files - What is the difference between OST and PST files?

Posted on 2012-09-13
Last Modified: 2012-09-28
There are OST and PST files used by outlook.  What is the difference?
Question by:brothertruffle880
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    Expert Comment

    Hi brothertruffle880,

    I think this is answer over here is informative:

    Hope it help,
    LVL 33

    Expert Comment

    It'll be mentioned in the above links, and you can understand the difference as mentioned below. Avoid, if this is repetitive information.

    OST file contains actual data that is downloaded from the mailbox server on to your local machine for accessing emails/calendar etc for offline viewing. In short, you can read your emails even if you unplug your laptop from all the network of your company.

    PST file is a file storage that is understood by Outlook for keeping a copy of your emails. Any email/calendar etc moved to a PST file will remove the data from your mailbox server and place it locally on your PST file.

    As always, if you have questions - feel free to ask.

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    Expert Comment

    All information above are correct.
    It's important have in known that OST (Offline Store) is a cache of the Exchange mailbox and PST (Personal Store) is a file that can contain messages, contact, ....
    You can take your PST file and read it on another computer (with Outlook), and you need to backup it to not loose information.
    You can't read a OST file on another computer, and you don't need to backup it, as it is a cache of your Exchange mailbox
    LVL 17

    Expert Comment

    Great info from other members here, just a few things to add.

    1. The OST file contains an encryption key that is unique to the user/mapi profile and the Exchange database the users mailbox resides in.  Therefore an OST file may ONLY be used by the user/account on the machine it was originally created.

    2. OST & PST files have a similar size limitation, i.e. for Outlook 2003 & 2007 the default format is UNICODE and the limit is 20GB, whereas in 2010 the UNICODE size limit is 50GB

    3. If the Exchange database fails and you do not have a DB backup you want can still recover the individual user data by having the users open Outlook and connect via OFFLINE mode, i.e. when connection is attempted by users they will be told the Exchange server is unavailable, however if desired they can connect OFFLINE which opens the OST file.  Then once opened they can Export all their data to a PST file making is portable and therefore usable on any system for recovery.  

    3.a  So if you have a DB failure and no backup or your backup is bad, you could then;
    i. Have each user open Outlook in Offline Mode and Export the data to PST

    ii. Delete, Move or Rename the damaged DB to an alternate location.  (Always good to g make a backup/copy even on damaged DB's since there is always a possibility of reading them via 3rd party utilities like our DigiScope product that can open offline databases to give you full access for export, recovery and discovery, but i digress..)

    iii.  Now mount the DB via Exchange Management and it will squawk and tell you that it cannot find database files and if you continue it will create a new DB.  Say yes and Exchange creates a new blank database and all users mailboxes can now send and receive email, i.e. so any backlogged email will flow into the respective mailboxes and once logged in to the account users can read and send new email however their historical data will be missing.

    iv. Now have each user open outlook and connect to Exchange so they can send and receive email.  Outlook will tell them they can connect to Exchange however since this is a new database the encryption key on this database is different once the user connects to the Exchange server the existing OST will be useless since the keys do not match and therefore a new OST file will be created.  So the main issue here is that you need to ensure that you have users connect to the OFFLINE OST and Export to PST before connecting to a new dial tone database.

    4. As mentioned above PST files can be read by another Outlook implementation, but OST files by default cannot be copied and read by another user on the same machine, nor on another machine by any user, however there are 3rd party tools that allow you to open OST files so another option you could do is if you have a failure you could copy the respective OST files to a safe place, dial tone the Exchange DB and then use a 3rd party utility to rescue data from the OST files.

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