Understanding Outlook PST and OST Data Files
There are two separate and distinct file types used by Outlook to store data
- PST files (Personal Storage)
- OST file (Offline Storage)
In the sphere of PST files, there are two formats – ANSI and Unicode. Outlook versions ’98 thru to ‘2002 (also known as Outlook ‘XP) can only create and read ANSI format PST files which have a maximum size limit of 2gb. Once the file exceeds that size, the file will no longer be usable and dealing with a file that has exceeded its max size can be problematic so the best approach would be to never let the file get close to its limit.
Starting with Outlook ‘2003 thru to all Outlook versions today, the Unicode PST file was introduced and used removing the 2gb size limitation. Unicode PST files cannot be read in Outlook ‘2002 or earlier but ANSI PST files can be read by any current Outlook version of Outlook.
ANSI PST files are not automatically upgraded when upgrading from an earlier version of Outlook so it is quite possible that even though you are using the latest version of Outlook having upgraded from a previous Outlook versions over time, you may still have a legacy ANSI PST file in use and be aware of it.
The way to verify the format of any PST file is to right-click on the root of the PST folder in the Outlook navigation pane –> Data File Properties –> click on the <Advanced> button. If the description in the File Format field indicates the format of the PST file:
- ANSI = Outlook Data File (97-2002)
- Unicode = Outlook Data File
ANSI PST file should be converted to the Unicode format if using any version of Outlook ‘2003 or later unless there is a very specific need to use the same file in an earlier version of Outlook. See “How to convert PST from ANSI to Unicode“
The most significant feature of each of these file types is:
- PST files can be be opened by any version of Outlook on their own (with exception noted above regarding Outlook ‘2002 or earlier and Unicode PST files)
- OST files can only be opened using the email account and Outlook profile used to create the file originally. In other words, you cannot open an .OST file on its own, such as would be the case if you deleted the email account used to originally create the file. Deleting and then re-adding an email account that uses an >OST file or adding the same email account in another Outlook profile does not allow you to use a pre-exisitng file.
Over time, things have become a little more confusing as to where Outlook stores the data for a given email account type. Today, the file type used for a given email account type (POP3, Imap, Mapi, EAS, Exchange) very much also depends on which Outlook version is being used especially moving into the realm of Outlook ‘2013 from an earlier Outlook version. It’s essential to know where Outlook stores its data when migrating from one version of Outlook to another especially if this migration is occuring on a new machine as opposed to being done as an “in place” upgrade.
The following is a simplified high-level overview of the types of data files used based on email account “Type” (POP, Imap etc) and version of Outlook being used. To get a more complete picture, the article “Understanding Default Folders versus Default Accounts” should also be reviewed along with the information contained in “Understanding Outlook Migration – Part 3 – Account Creation“
PST files will be created in the following circumstances
- POP3 Email Accounts – all Outlook versions
- POP3 accounts use a PST file for data storage data each of which can utilize its own PST file or be configured to use a single PST data file.
- IMAP Email Accounts – Outlook versions ‘2000 thru ‘2010
IMAP Email Accounts – Outlook ‘2010 and earlier
Two PST files will be created.
– One labeled as “Outlook Data File” which will contain the default folders for contacts, calendar and tasks etc. This PST file will also have email folders for <Inbox> etc in the tree but those will not be used. The additional folders are simply created as a by-product of a PST file being designated as the “default” data file
– The second one will contain the email folders used for the account. If the email account is deleted and re-added (or this PST file is deleted), a new PST file for this info will be created. You cannot re-use a pre-existing PST file. Once the email account is syncronized with the server, it will contain the same information as found on the server
*** if another data file is already configured in the profile and designated as the “default” – the first PST file will not be created in this instance.
IMAP Email Accounts – Outlook ‘2013
Starting with O’2013, Imap accounts utilize an .OST (offline) storage file. If the file is deleted or the email account is deleted/re-added, a new OST file will be created.
If this is the first and only account in the profile, the folders for contacts etc will appear in the OST based folder tree with the added description “This Computer Only”. If the OST file is deleted and/or you delete/re-add this email account, you will lose all information contained in the folders with the “This Computer Only” designation”. It is essential that you create and maintain your own backup of this information should it be required in future.
*** if another data file is already configured in the profile and designated as the “default” – the “This Computer Only” folders will not be created in the OST folder tree
MAPI Email Account (Outlook.com) – Outlook ‘2010 & earlier using Outlook Hotmail Connector
Uses an .OST file which will get recreated if the file is deleted or if the account is deleted/re-added to the profile. The data file created for this email account type will always contain the folders for Contacts/Calendar etc whether or not there is another data file designated as the default for the profile since the information in these folders is synchronized with the Outlook.com server.
EAS (Exchange ActiveSync) Email Account (Outlook.com) – Outlook ‘2013
Same as when configured as a MAPI account using the Outlook Hotmail Connector in earlier Outlook versions. However, there are some distinct differences in terms of functionality between the MAPI and EAS protocols – see “Understanding Email Protocols“
Microsoft Exchange Email Account – hosted or local – all Outlook versions
Uses an OST file which will get recreated if deleted or the account is deleted/re-added in the Outlook profile.
Category: Understanding Outlook
