Whenever you use or create an email address in Microsoft Outlook, all the data within that email account is contained within a .pst (Personal Folders file). This is a database-type file that Outlook uses to keep everything organized. An .ost, Outlook Data File is essentially a mirror of your Exchange mailbox held locally on your computer.
As email is quite important to all of us as a communications medium, I thought I would share everything I know about these files. I refer to Outlook here as this is the default email client for these files. Other email clients do work with .ost and .pst files so if you use something else, just refer to that instead of Outlook.
There are two types of Outlook Data File. The .pst, Personal Folders file and .ost, Outlook Data File. A .pst file is primarily for home users and anyone who uses Outlook Web Access through Outlook.com or Hotmail.com. An .ost file is for small to medium-sized businesses that use Microsoft Exchange.
Personal Folders file
The Personal Folders file (.pst) is like a database of all your emails, attachments, replies, calendar entries, contacts and all the contents of your email inbox. If you use Office 2013/16 without Exchange, you will have one or more .pst files on your computer. One .pst file is created for every POP3 mailbox you have in Outlook.
The .pst file will be stored in C:\Users\USERNAME\Documents\Outlook Files.
The .pst file is a copy of your email data that is permanently stored and updated on your computer. Should you need to, you can copy the .pst file onto another computer to set up email and everything would work if you import it into a different copy of Outlook or other email client.
Outlook Data File
The Outlook Data File (.ost) is for users of Microsoft Exchange IMAP email accounts. This is usually businesses that utilize either Office 365 or Office 2013/16. The Outlook Data File is also often referred to as the ‘Offline Data File’ as that is its purpose. To allow users to manage emails even if they don’t have a live link to Exchange. You will still need Exchange to send and receive emails but read and replying can all be done offline and are then sent by Outlook once connected.
The .ost file is stored in C:\Users\user\AppData\Local\Microsoft\Outlook
An Outlook Data File is a mirrored copy of your Exchange email data. So rather than being the main copy of your inboxes like the .pst, an .ost is only a copy. If it gets lost or damaged, Outlook will automatically download a fresh copy from Exchange and continue working.
Unlike the .pst file, you ideally should not move or import an .ost file. You have to create a new one within the instance of Outlook you are using at the time. If you needed to migrate email to a new computer, you would need to set up the email account from scratch and let Exchange download a new file to that computer.
Copying, storing and deleting
There are some computer repair companies that don’t back up .ost files because they can be quickly downloaded from an Exchange server. What’s the point in adding another location to a backup routine when you don’t need to right? Wrong. At least in my opinion.
What happens if something goes wrong with the local computer and the internet connection? What would happen if the Exchange server got hacked or went down? What id Exchange caching hasn’t been set up? Having a copy of the .ost file and recovering it means the user could still be productive while connectivity is restored. Otherwise, the user cannot do anything with email until that time.
I would always suggest backing up .ost files as part of your standard backup routine. While the files themselves can grow up to a few hundred megabytes, if you depend on email to get work done, it makes sense to back it up. Plus, if the person who set up your email didn’t enable Cached Exchange Mode, which is a synchronized copy of your email on your computer, you could lose a lot of work if you lost the file.
Exporting .ost files
As mentioned above, you can copy and move a .pst file as you see fit. Any copy of Outlook and most other email clients will pick it up and run with it without issue? So what about an .ost file?
Microsoft suggests you create a new .ost file rather than importing an existing one but it is possible to do. There is a procedure whereby you disable Cached Exchange Mode and create a standalone .pst which you can import into another instance of Outlook should you need to. This page on the Microsoft Office site shows you how. If the new computer has a live connection to Exchange, there should be no need to import a .pst file but the option is there should you need it.
What happens if you delete a .pst or .ost file?
I, and the team here at Dave’s Computers have dealt with many dozens of users who accidentally deleted their .pst or .ost files when housekeeping or trying to free up disk space. While an inconvenience, it isn’t terminal.
If you accidentally delete a .pst file, you can potentially restore it from the folder itself. Navigate to C:\Users\USERNAME\Documents\Outlook Files, right click on the folder and select ‘Restore previous versions’.
If you accidentally delete an .ost file, you should be able to get Exchange to download a fresh copy.
- Open Outlook and select File.
- Select Account Settings and the email address in question.
- Select the Data Files tab and the email address in question.
- Select Settings and the Advanced tab in the new window.
- Uncheck Use Cached Exchange Mode and hit Apply.
- Close all the windows and shut down Outlook.
- Restart Outlook.
When Outlook restarts, it should automatically connect to Exchange and download a new copy of your .ost file. Repeat the above process to enable Cached Exchange Mode once more to complete.
So that’s my guide to Microsoft PST and OST files. There is a lot more to running Outlook and Exchange but that is an overview of what the tow files are, what they do and how they enable you to use your email. Hope the information helps!