Fixing your Outlook Data Files (PST and OST)

Outlook Data Files (PST and OST) are one of those things you don’t worry about when setting up Outlook for Windows for the first time.  But as you use Outlook more, it’s important to understand what Microsoft is doing for backup and fixing / repair purposes.

The main data files (.PST or .OST) are a store on your computer of email messages, calendar information, contacts, and tasks.  Each account has it’s own PST/OST file.  This only applies to Outlook for Windows – not Outlook for Mac.

In modern cloud mail storage (, Gmail etc.) the local copy is a lot faster to access than constantly referring to the online store.  You can access your email etc. when there’s no Internet access at all.

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.

PST and OST files are indexed by Windows so they can be quickly searched.

Let’s see the difference between the two Outlook data files.

PST data file

For POP and IMAP accounts, a PST Outlook Data File (.pst) contains your messages and other Outlook items on your computer.

Outlook moves your email messages etc from your mail server and saves them on your computer.

Using a (pst) file you can export or back up items like your email messages, calendar info, contact, and task data from your email account, and can also import your data from another (pst) file.

It’s VITAL that you backup your PST files because they are the only copy of your information.  If the PST is badly corrupted or the computer lost, then you’ve lost it all.

Everyday Backups has a chapter devoted to backups of Outlook data.  PST, OST and the other places that Outlook saves settings.

OST data file

OST files were originally for Exchange Server accounts only.  These days they are used by other mail services that have Microsoft’s ActiveSync technology AND IMAP connections.

Account types such as Microsoft 365 email hosting, Exchange accounts, and accounts all use an Offline Outlook Data File (ost) file.

The most common IMAP connection is to Gmail free accounts.

OST’s have a synchronized copy of your mailbox information on your local computer. Even if 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.

Save more offline

Recent versions of Outlook for Windows will only synchronize recent data, leaving older data on the server.  That’s great if you have limited disk space, If you have plenty of disk space (most desktop and laptop computers will) then it’s better to synchronize everything.  The ‘Mail to keep offline’ setting to ‘All’.

You’ll access and search your ‘life’ a lot faster and be less reliant on Internet access.

BIG, really big

PST and OST files can get big, really big.  They can grow to saving many years of email, contacts, appointments etc.  That’s quite normal and nothing to worry about.

Outlook Data Files of 20GB or more are quite common these days.

In the early days of Outlook, PST/OST’s were limited to less than 2GB.  Those days are long gone and now Outlook data files can theoretically grow into many Terabytes (yes TERAbytes).

Where are the Outlook PST or OST files?

Locating your Outlook Data Files is easy. 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
  • In right pane, click Account Settings, and then click Account Settings.

  • Click on Data files

  • On the Data Files tab, click Open Folder Location.
  • You can see the .ost data files saved in this location.

Alternatively, the offline data files can be navigated from the default pathway: drive:\Users\<username>\AppData\Local\Microsoft\Outlook.

Outlook Data Files (pst) created by using Outlook 2016 are typically saved on your computer in the Documents\Outlook Files folder. Most data remain on the server; however, any items like Calendar, Contacts and Tasks that are saved locally should be backed up.

Why should you fix the OST And PST Files?

This is a good to have the option. If you encounter an Outlook crash or come across error what would you do? Fixing your Outlook Data files can help you recover Outlook pst file with the help of offline Outlook data file. If you have lost the data on your local laptop then you can restore from OST files.

Fix for Outlook PST and OST data files

PST/OST files are very large and very complicated database files.  Because of their size and complexity, errors can creep in.  Most of those errors aren’t serious or even noticeable.

But sooner or later, the files may stop working or, more likely, you’ll have search/ indexing problems. Which is why Outlook includes the Inbox Repair tool.

Now we reach one of those situations where must explain two different ways to do something.  The official Microsoft approved method and the method that really works.

Outlook for Windows is installed with an Inbox Repair tool ( SCANPST.exe ) which can analyse and repair your PST and OST files.

Yes, OST files can be scanned and repaired with the ScanPST.exe tool.  There was a separate ScanOST.exe but that’s been dropped.  What Microsoft doesn’t say is that ScanPST works with OST files.  We’re sure of this because we’ve used ScanPST.exe on many OST files.

Repairing a (PST or OST) file:

Before running the SCANPST.exe program make sure you have enough free disk space.  You need enough space for a complete copy of the original PST/OST file plus ‘working space’ for the repair.  We suggest at least free disk space at least double the scanned PST/OST file.  For example, for a 15GB Outlook file you should have at least 35GB of free disk space before ScanPST.exe is run.

  • Close Outlook.
  • Open File Explorer (Windows key + E).
  • Browse the following path:

C:\Program Files (x86) \Microsoft Office\root\Office16 (or whatever applies for earlier versions of Outlook eg \Office15 \Office14 etc.)

  • Inside the folder, find and double-click the SCANPST.EXE file.
  • Click the Browse button and locate the pst file you want to repair.

The tool will find the normal PST folder automatically.  You’ll have to find the OST files yourself, it’s best to locate the OST from Accounts Settings | Data Files | Open File Location.

  • Click the Start button. Before you press Start, remember to close the Outlook application, if you still have it running on the background.

  • That will commence a check of the PST/OST to see if there’s any problems.  It may take a while.
  • Once the initial scan is over, you’ll see a message about any errors or ‘inconsistencies’ to be repaired.

  • Click the Repair button.

Repairing may take some time.  Set it running before lunch or overnight.

  • After Outlook has repaired the error, you will see a notification on the screen saying repair complete. Press OK.

Need more?

Sometimes that’s not enough.  You might need to run ScanPST.exe over the same data file many times to fix all the errors.  Very occasionally, there are problems or errors that refuse to go away.

Fixing an Offline Outlook Data File (ost) – Official method

This is Microsoft’s approved method of ‘fixing’ an OST.  In short, they want you to totally delete the existing OST file then let Outlook totally recreate it from online storage.

It’s an easy fix for Microsoft to recommend but impractical in the real world.  Totally remaking an OST is a heavy burden on the computer and server.  Ideally, you’d do this on the same local network as the mail server, for the fastest data transfer.  However, with many mail servers either offsite or in the cloud, it can take many hours and a lot of internet bandwidth to download many gigabytes of data.  In many parts of the world, that’s a slow or costly burden.

If you have an Outlook 365, or Exchange account, you can follow these steps to fix problems with a (ost) data file in Outlook:

  • Close Outlook and make sure it’s fully closed.
  • Open Control Panel.
  • Click on User Accounts.
  • Click on Mail.
  • Using the Mail Setup experience, click the Show Profiles
  • Select the profile with the account you want to fix. (In Outlook 2016 the default profile is called “Outlook.”)
  • Click the Properties
  • Click the Email Accounts
  • Click on the Data Files
  • Select the account to be repaired.
  • Click the Open File Location
  • Before you continue, make sure to close the “Accounts Settings,” “Mail Setup,” and “Mail” windows.
  • Right-click the .ost file for the account with problems and select Delete.
  • Once you complete these steps, open Outlook again to automatically create a new .ost files for that particular account.
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