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The Offline Storage Table, or OST, is a file format that is used by Microsoft Outlook to store the user’s information in an offline location. It saves emails, tasks, contacts, notes, and calendar in OST files. This makes sure that the users have access to all the data even at times of no internet connectivity. On the other hand, an EML file is an electronic message that contains a single email along with its other contents like the message, date, subject, receiver/sender details, and attachments. It is used by several email clients, including MS Outlook, Thunderbird, Zoho Mail, Windows Live Mail, Apple Mail, etc.
Scenarios like saving specific emails or making emails platform-independent (for legal needs) trigger the need to convert emails from OST to EML format. Moving forward, we will discuss how you can save emails from a connected OST as well as from an orphaned OST file. But before that, let’s discuss a few more reasons for the conversion.
Reasons to Convert OST to EML Format
There are various reasons behind users needing to save OST to EML format, some of them are:
- Mange OST file size: A large OST file can cause problems like Outlook freeze or failure in receiving emails. Exporting emails to EML helps to fix oversized OST file issues.
- Backup needs: In case of any system failure or accidental deletion, users can take advantage of the different methods to take backup OST files in EML format.
- Data sharing: OST files cannot be shared with others easily. While using an EML file, the transfer becomes simple and quick and further ensures smooth communication.
- Switching to another platform: As mentioned earlier, the EML format is compatible with multiple platforms, while OST compatibility is limited to Outlook. Hence, conversion allows you to access Outlook emails in other email clients like Thunderbird.
After analyzing the reasons, let’s move towards the ways to export emails from OST to EML format.
How to Convert OST to EML Format?
The method to save OST to EML format can be done in two situations. Either the OST file is connected with the Exchange Server, or the OST file is not connected with the Exchange Server. Go through the scenarios below and follow the steps carefully.
When the OST file is connected with the Exchange Server
For a connected OST file, use the following 3 ways to convert OST to EML:
Method 1: Save OST emails to EML using the Outlook web version
This method is platform independent, therefore anyone can use it. Whether you are on Mac, Windows, or Linux, you can use the Outlook Web App (OWA) to download and save any email with attachments and metadata in .eml format. Steps to do so are:
Step 1. Navigate to outlook.live.com and log in with the user whose emails you need to convert.
Step 2. Open the email. Click on the 3-dot icon. Go to Download and select the Download as EML option.
Method 2: Save OST emails as EML using the new Outlook
Unlike the classic Outlook, new modern Outlook desktop app version allows you to save emails to EML. The steps to convert emails from OST to EML are:
Step 1. Open Outlook and configure the source email account (if not added).
Step 2. Right-click an email, go to Save as > Save as EML.
Method 3: Transform OST emails to EML on Mac
If you are using Mac OS and want to convert OST to EML, you can do it with Outlook for MAC with these steps:
Step 1. Open Outlook for MAC and select the emails to convert.
Step 2. Click File and select the Save As option.
Step 3. Provide a name for the email, specify where to save, and make sure the saving format is Email Message. Click Save to save the email as EML.
Method 4: Use Thunderbird to save emails from OST to EML
For this method, you can use any email client that supports an IMAP account and allows exporting emails in EML file format. I am using Thunderbird here. You can configure your Outlook account in Thunderbird and export emails as .eml with the following steps:
Step 1. Open Thunderbird. Click on the menu button. Select Account Settings > Account Actions > Add Mail Account. Enter the credentials of your Outlook account, click Continue, and follow the login process.
Step 2. After adding the account, select the email (single or multiple). Right-click and select Save As.
Step 3: Choose where to save the emails and specify the value of the Save as type option as Mail Files (*.eml). Finally, click Save.
Read Also: How to Import OST to Thunderbird?
Limitations of manually converting OST to EML
Manual methods are free but come with limitations:
- Works only if you have a connected & healthy OST file. Damaged OSTs will require OST file repair before any manual method can be used.
- Not suitable for converting bulk emails from OST to EML, it will be very time-consuming.
- Success depends on multiple factors, like the internet, an email client (Outlook or Thunderbird), and a working Exchange Server.
When the OST file is not connected to the Exchange Server
A professional OST converter tool is recommended to save Outlook emails in EML format when the OST file is disconnected from the Exchange Server. Outlook cannot read an offline OST file, and thus, manual conversion is out of scope.
A reliable tool that can read your orphaned, disconnected OST file and save emails to EML is the Kernel for OST to PST Converter tool. It is specially built and engineered to access offline Outlook files (OST) and save their emails in EML and several other formats. A few advantages of the tool are:
- Allows to convert single as well as multiple OST files while maintaining the integrity of the data.
- Built-in filters to convert OST to EML files selectively.
- Compatible with all versions of Microsoft Outlook and the Windows operating system.
Steps to save the OST file to EML format using an automated tool
You can easily save your OST files to EML format using OST to PST Converter with these steps:
Step 1. Install and open the tool. Click on the Browse button to select the OST file that you want to save in EML format. 
Step 2. After selecting the file, you can preview its content before saving it to EML format. To convert, select the folders and click Save.
Step 3. Apply filters and click Next.
Step 4. Select the desired location to save the EML file and click on the Finish button to complete the process.
Conclusion
For Windows users, the quickest way to convert OST to EML is to use the new Outlook. Mac users can use Outlook for Mac. Alternatively, you can use Outlook Web App (this method even works on a Linux system). But let me remind you, if you want to save the emails from an orphaned OST file, you must use the recommended OST to PST converter tool, as no manual method supports offline OST files.
Frequently Asked Questions
Ans. Manual methods like new Outlook and Thunderbird support limited bulk conversion. For full folder or large-scale batch conversion, a dedicated OST converter tool will work better.
Ans. Yes, EML files store the complete email structure, including attachments, sender/receiver details, timestamps, and subject.
Ans. An OST file that is no longer linked to its Exchange Server account is called an orphaned OST file. This is mainly caused by account deletion, server migration, or profile corruption. In such cases, OST recovery is required to access the data stored in the file.
