When you add an email account to Outlook, a local copy of your information is stored on your computer. This feature allows you to access your previously downloaded or synchronized email messages, calendar information, contacts, and tasks without an internet connection.
Note: You cannot import from an Offline Outlook Data File (.ost) file. Because these files are saved on your computer, they aren’t subject to mailbox size limits on a mail server. By moving items to an Outlook Data File (.pst) on your computer, you can free up storage space in the mailbox on your mail server. The basic difference between OST and PST is that OST files are used to save data for offline use and are stored in MS Exchange Server. Whereas PST files are personal folder data and stored on client’s hard disk.
Certain types of accounts, such as POP accounts, store their information in Outlook Data Files (.pst files). Outlook 365 accounts, Exchange accounts, IMAP accounts, and Outlook.com accounts store their information in Offline Outlook Data Files (.ost files).
Outlook Data File (.pst)
An Outlook Data File (.pst) contains your messages and other Outlook items and is saved on your computer. POP accounts—which is a common type of account you can create when adding an email account from an internet service provider (ISP) like Xfinity or AT&T or Cox—download all of your email messages from your mail server and save them on your computer.
Tip: If your Outlook Data File (.pst) becomes large it can slow down your OneDrive sync and may show 'Processing changes' or 'A file is in use'. Learn How to remove an Outlook .pst data file from OneDrive.
Note: In Outlook 2013 and earlier versions, IMAP accounts also used an Outlook Data File (.pst). In Outlook 2016 and Outlook for Office 365, IMAP accounts use Offline Outlook Data Files (.ost).
You can also use an Outlook Data File (.pst) to backup or export items from any type of email account. For more information on backing up your email messages, calendar, contact, and task data, see Export or backup email, contacts, and calendar to an Outlook .pst file. You can also import email messages, calendar, contact, and task data from another Outlook Data File (.pst). For more information about importing items, see Import email, contacts, and calendar from an Outlook .pst file.
Note: You cannot import from an Offline Outlook Data File (.ost) file.
Because these files are saved on your computer, they aren’t subject to mailbox size limits on a mail server. By moving items to an Outlook Data File (.pst) on your computer, you can free up storage space in the mailbox on your mail server.
When messages or other Outlook items are saved in an Outlook Data File (.pst), the items are available only on the computer where the file is saved.
Offline Outlook Data File (.ost)
Most other account types, such as IMAP accounts, Office 365 accounts, Exchange accounts, and Outlook.com accounts use an Offline Outlook Data File (.ost) file to store a synchronized copy of your mailbox information on your local computer. When your connection to the mail server is interrupted, for example, when you're on an airplane without WiFi or if your internet connection disconnects, you can still access all emails, calendar data, contacts, and tasks that have been previously downloaded. You can read and compose messages, create appointments, and add contacts. However, your email messages won't be sent and no new messages will be downloaded until your internet connection is restored.
When the connection is restored, changes are automatically synchronized, and the folders and items on the server and on your computer are identical again.
Outlook Group Storage File (.nst)
The first time you access a Group in Outlook, a new type of Outlook data file is created. This file has an extension of .nst, and stores your Groups conversations and other local Groups data. All of this information is synchronized between your computer and the email server, so there is no need to back up this file or import it when you set up Outlook on a new computer.
File locations
The fastest way to open the folder where your Outlook Data Files (.pst and .ost) are saved is to do the following:
In Outlook, click the File tab.
In right pane, click Account Settings, and then click Account Settings.
On the Data Files tab, click an entry, and then click Open Folder Location.
Outlook Data Files (.pst) created by using Outlook 2013 or Outlook 2016 are typically saved on your computer in the DocumentsOutlook Files folder. If you upgraded to Outlook on a computer that already had data files that were created in Microsoft Office Outlook 2007 or earlier, these files are saved in a different location in a hidden folder at drive:UsersuserAppDataLocalMicrosoftOutlook.
Tip: If your Outlook Data File (.pst) becomes large it can slow down your OneDrive sync and may show 'Processing changes' or 'A file is in use'. Learn How to remove an Outlook .pst data file from OneDrive.
The offline Outlook Data File (.ost) is also saved at drive:UsersuserAppDataLocalMicrosoftOutlook. Most data remains on the server; however, any items that are saved locally should be backed up. For example, Calendar, Contacts, Tasks, and any folders marked Local only.
Tip: For information about hidden folders in Windows, see Windows Help and Support.
Related Topics
Create an Outlook Data File (.pst) to save your information
Import email, contacts, and calendar from an Outlook .pst file
Repair Outlook Data Files (.pst and .ost)
Hi Veasna,
Thank you for posting in Microsoft Community.
Both OST and PST files are Outlook data files. OST file is a mirror image of your web-mail data file. The email account types like IMAP account, MAPI account Exchange Account create an OST file to save data when configured in Outlook. PST file is an Outlook data file while saves data locally on the computer.
For more information on Outlook data files you may refer to the link mentioned and check:
Q1: The Outlook data file ends with (1), (2) etc. because these are different data files. If you have configured the same email account multiple times to differentiate between the data files Outlook automatically adds the suffix (1) or (2).
If you do not need those data files then you may delete those data files.
Q2: If you have a POP account or if you have created a PST data file then it should be saved on the system.
PST file location for Outlook 2010:
If you had upgraded from earlier version of Office then the data file will be save din the location mentioned:
C:Users<username>AppDataLocalMicrosoftOutlook
Otherwise by default the PST file will be saved in the location:
C:Users<username>DocumentsOutlook Files
You may also search for the PST files saved on the system from the explorer:
Note: In the search box you may type *.pst to find all the files with pst extension.
Q3: In Outlook 2010, when you configure an email account by default the OST and PST files will be saved in the same location (C:Users<username>DocumentsOutlook Files ) which is thedefault location to save Outlook data file.
If OST and PST files are not saved in the same location then open Outlook, check the location where the PST file is save, remove the PST file from Outlook, browse to the location where the PST file was saved, copy the PST file, save the PST file in the location where OST file is saved, open Outlook and open the PST file in Outlook from its new location.
To remove the PST file from Outlook:
- Open Outlook.
- Click on File< AccountSettings< OutlookData Files tab.
- Note the location where the PST file is saved, highlight the PST file and click on Remove.
To open the PST file in Outlook:
- Open Outlook.
- Click on File Open< OutlookDataFile.
- Browse to the location where you have saved the PST file highlight it and click on open.
Ost Pst Files
If you have associated any email account with the PST file then follow the steps mentioned.
- Open Outlook.
- Click on File< AccountSettings< Email tab.
- Highlight the email account associated with the PST file, click on Changefolder and point to the right Outlook Data file.
Where Is My Pst File
Q4: refer the link mentioned to backup the Outlook data file:
Note: To backup the OST file, you may export the OST file (the complete data file) in PST format.
Ost Or Pst Files
Q5: Ensure to backup the data file which contains all your data. If the data file is open in Outlook then to check the location of the data file:
- Open Outlook,
- Click on File Open< OutlookDataFile.
- Under Location check where the data file is saved.
I hope the above information helps. Let us know if you need any further assistance.
Create Pst Files
Thank you.