How to convert OST to PST (with or without Outlook)

What is the best OST to PST converter software?

The short answer is… Aid4Mail. Compared to competing tools:

  • Aid4Mail is much faster.
  • Offers more control over the conversion process.
  • Recovers mail from corrupt OST files.
  • Handles very large OST files reliably.

If you don’t have Aid4Mail yet, you can experiment with the free trial version or purchase a low-cost 2-week Aid4Mail Converter license. You’ll be up and running in minutes.

What is an OST file?

An OST file is created by Outlook for Windows when you connect to an Exchange or IMAP account. It acts as local storage to speed up mailbox access and reduce bandwidth usage. By default, it contains messages that are less than a year old.

An OST file no-longer associated with an Outlook profile is an “orphan”. This is because it can’t be opened by Microsoft Outlook. However, when converted to a PST file, the mailbox data becomes accessible again. By Outlook and also by other applications.

What can Aid4Mail do?

Aid4Mail can convert your OST file to a PST with or without Microsoft Outlook installed. However, you will get better performance when Outlook is present. Aid4Mail can also faithfully restore your folder structure.

Instructions for converting an OST file to PST with Aid4Mail.

Please follow the steps below. For any setting that’s not specifically mentioned, you can either use the default (preset) value or consult the Aid4Mail 5 User Guide for details. You can open it from Aid4Mail by pressing the F1 key.

Step 1: Open Aid4Mail’s settings

Aid4Mail settings showing the session name set to "John Doe OST to PST". The Source and Target sections are expanded but not set. The Filter section is not expanded.

  1. In Aid4Mail, select the Settings tab.
  2. Update the Session name so it describes the task at hand. In this example, we will be converting John Doe’s OST file to a PST file so we’ve entered John Doe OST to PST as the session name.
  3. Below the session name there are three sections: Source, Target and Filter. Each can be displayed or hidden by selecting the +/- symbol next to the section heading. This tutorial requires them all to be displayed.

Step 2: Set an OST file as your source

Source settings for the OST format.

  1. In the Format list, select OST file.
  2. Specify the location of your OST file. You can write it directly in the field or use the Select button to browse your file system. By default, the Windows version of Microsoft Outlook stores OST files in this location:
    C:\Users\<WINDOWS USERNAME>\AppData\Local\Microsoft\Outlook\
  3. Choose whether to Remove journaling envelope if your email files are journaled.

Step 3: Set a PST file as your target

Target settings for the PST format.

  1. In the Format list, select PST file.
  2. Choose whether to Process using Outlook. This means using Outlook’s internal engine (MAPI) to create the PST file. Using Outlook’s engine is faster but requires Outlook to be installed on the same computer as Aid4Mail. If you don’t have Outlook installed, keep this option off.
  3. Specify where you want your PST file to be saved. You can write directly in the field or use the Select button to browse your file system. You can also set a Password to protect the file.
  4. Optionally, set a folder Location in PST file. This is a folder inside the PST file where your converted mail will be saved. In our example, we have set a folder called Converted mail. This folder doesn’t exist yet but Aid4Mail will create it.
  5. Select an option for the Folder structure of the exported mail. Usually you’ll want to keep it the same as the original, so choose Same as source folder structure.
  6. Decide whether to Limit PST file size. In our example, we’ve set it to 50GB which is the maximum file size allowed by Outlook by default. Once the file has reached its limit, newly converted emails will overflow into a second PST file. And when that one’s full then into a third PST file, and so on.
  7. If you want to add to an existing PST file, choose Append messages to target file if it already exists.
  8. Add extra metadata to email headers when your chosen target format or settings would otherwise result in data loss. For example, if the target format doesn’t support some of the source format’s header fields. Or if the original folder structure would be lost due to your settings. With this option, the data is not lost but instead added to the email headers.

Step 4: Set your filter (optional)

Filter settings showing a folder selection.

  1. Under Folder filtering, pick Folder selection. Your OST file’s internal folder structure will appear in the box below.
  2. Select the folders you want Aid4Mail to process. The others will be skipped.

Step 5: Run your OST to PST conversion

Session > Run menu command. The Run button is also visible on the button bar.

Select the Run button or Session > Run from the menu. Aid4Mail will convert your OST file to PST. It will automatically open the Progress tab so you can monitor what’s going on. Once finished, you will find full reports in the Progress log and, if relevant, the Error log.

Step 6: Open the PST file in Outlook

You can do this by selecting File > Open > Outlook Data File from Outlook’s main menu (older versions of Outlook may use different menu names).

That’s it!

We hope these simple steps to convert OST to PST were helpful.

If you’re looking for further information, take a look at the FAQ below. You can also consult our knowledge base articles and the Aid4Mail 5 user guide. If you still have questions, don’t hesitate to contact our award-winning Helpdesk.

If you’d like to buy an Aid4Mail license, please visit the online store.

Or if you’d like to Aid4Mail first before purchasing, download a free trial and run your own tests. We want you to have full confidence in Aid4Mail!


What are OST and PST files?

OST and PST are both Microsoft Outlook data files. OST files are typically used to store offline data from an Exchange server or Microsoft 365 (formerly Office 365), or IMAP account. PST files were formerly used for this purpose in older versions of Outlook and are still used to store offline POP account data.

What is an “orphan” OST file?

An OST file that is not associated with an Outlook account.

Why convert an OST file to the PST format? 

The main reason is that PST files can be imported into MS Outlook and OST files cannot. This is true for other email-related applications too and makes PST files good for:

  • Backing-up Outlook data
  • Long-term archiving
  • Data-exchange

How to convert an Outlook OST file to PST?

Please follow the step-by-step instructions earlier in this article.

Are PST files widely used?

Yes! PST is the most common target format among Aid4Mail customers. It is used in:

  • Business
  • Education
  • Forensic and investigative work
  • Litigation
  • And more…

What is Aid4Mail?

Aid4Mail is a user-friendly Windows application that specializes in processing email: converting it from one format to another, searching it, and exporting it in useful ways.

Why use Aid4Mail to export OST to PST?

There are a few manual methods to convert an OST file. Aid4Mail stands out from the crowd because its conversions are reliable and avoid data loss. Aid4Mail can recover mail from an OST, even if it’s damaged. And it can export to a whole range of file formats, email applications and services.

Where can I get Aid4Mail?

You can download a free trial of Aid4Mail from our downloads page. Licenses can be purchased from our online store.

Where can I get help?

Aid4Mail is installed with a a very comprehensive, context-sensitive user guide. Just press the F1 key while using Aid4Mail to open it.

There is also a whole range knowledge base articles and tutorials on our website. If you’re still stuck, contact our award-winning Helpdesk.

About Fookes Software

Fookes Software Ltd
La Petite Fin 27
1637 Charmey (en Gruyère)

For over 25 years we have been developing award-winning tools and productivity software. We also have more than 20 years of expertise in the field of email processing and analysis.

Our clients include Fortune 500 companies, government agencies, law firms, universities, and professionals specializing in e-discovery and forensics from around the world.

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