Understanding the difference between PST and OST files
Every email account configured in Outlook has a data store connected to it which is either a PST or OST data file.
So what’s the difference?
A PST (Personal Storage) file is one that permanently stores on the local hard drive. It never gets deleted by Outlook and can be copied to other installations of Outlook when upgrading or simply moving to a new machine.
An OST (Offline Storage) file is a “mirror” of the data that is stored on the server for the particular email account. If Outlook cannot find the file when it’s opened, a new OST file will be created and the data on the server will be sync’d down to the OST file. If Outlook is offline, you can still access the information that has already been sync’d (downloaded) from the server. Once Outlook gets connected to the server, any changes made (or new emails sent) while offline will be sync’d with the server.
OST files can only be used by the Outlook profile/email account that originally created it. You cannot reuse an OST file nor can you dictate what type of data file will be associated with an account.
What file type does each email account type/Outlook version create?
POP3 accounts
A single PST file is created for POP3 accounts in all versions of Outlook without exception
IMAP Accounts
In OL’2010 and earlier, there are two PST files created for the first IMAP account configured (this assumes that no other account has already been configured). One contains the email data only and the other is designated as the default data file for the profile which contains the default contact/calendar folders. If Outlook cannot find the PST file that contains the email data, a new PST file will automatically be created and sync’d with the server. The default PST data file containing the default contact/calendar folders is a local data store only. None of this information ever get’s sync’d with the server and the file never gets deleted from the system by Outlook
In OL’2013+, a single OST file is created by Outlook which contains both the email folders that get sync’d with the server while also containing contact/calendar folders that only store information locally. These folders have the added description of “This Computer Only” added to the folder name.
Outlook.com/Exchange Accounts
A single OST file is created which is a mirror of everything that is stored on the server including contact/calendar information. If this file ever gets deleted or not found, then Outlook will automatically create a new OST file and sync all folders with the server.
Caveats about OST files
If an IMAP account is deleted in Outlook ‘2013+, the corresponding OST file is also deleted. That means that unless the data stored in any folder with “This Computer Only” in the folder name has been backed up, it will be irretrievably be lost.
Cannot make a copy of an OST file and then open it on its own in Outlook. It can only be accessed by the original Outlook profile/email account that created it. If you attempt to configure the same email account in the same Outlook profile a second time, you cannot designate a prior OST file to be used for the account, a new OST file will always be created
If you have an “orphaned” OST file and want to retrieve the data inside it, the only recourse is to use a 3rd party OST2PST conversion tool. See: How to fix corrupt Outlook ost files
Category: Understanding Outlook
