EML vs MSG: Key Differences & How to Convert (2026 Guide)

You've just exported a critical email, but now you're staring at two file formats—.eml and .msg—and you have no idea which one will save your metadata or open on your colleague's computer. I've been there. In fifteen years of handling email migrations and archival projects, I've watched IT teams make the wrong call and lose crucial message properties in the process. The difference between EML and MSG isn't just about file extensions; it's about how you work, who you share with, and how long you plan to keep those messages around.

This guide gives you a scenario-based framework for choosing between the two formats. We'll look at what each format does well, where they fall short, and exactly how to convert between them when you need to. By the end, you'll know which format fits your workflow—not just which one is "better" in the abstract.

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What Is the EML File Format? An Open Standard for Universal Access

EML is the closest thing email has to a universal language. It's a plain-text file that follows the MIME (Multipurpose Internet Mail Extensions) standard, specifically RFC 822 and its successors like RFC 5322. When you save an email as EML, you're essentially storing the raw message—headers, body, and attachments—in a format that any email client can read.

I remember the first time I opened an EML file in a basic text editor. There it was: the entire email laid bare, from the From: and To: headers down to the Base64-encoded attachment data. That transparency is exactly why EML has become the default choice for email archiving and cross-platform sharing.

Key Characteristics and Components of EML Files

An EML file has a predictable structure that separates metadata from content. Here's what a typical EML file looks like when you open it in a text editor:

From: sender@example.com
To: recipient@example.com
Subject: Project Update
Date: Mon, 22 Dec 2025 10:30:00 +0000
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="boundary123"

--boundary123
Content-Type: text/plain; charset="utf-8"

Message body content here.

--boundary123
Content-Type: application/pdf; name="report.pdf"
Content-Transfer-Encoding: base64

[Base64 encoded attachment data]
--boundary123--

The core components are straightforward:

• Headers: Standardized fields like sender, recipient, subject, and date
• Body: Plain text, HTML, or both in a multipart arrangement
• Attachments: Encoded as MIME parts, typically using Base64
• Boundaries: Markers that separate different sections of the message

Because EML is text-based, you can always inspect it, even if the email client you're using doesn't render it perfectly. That's a lifesaver when you're debugging a migration or verifying that an archived message is intact.

Benefits and Advantages of the EML Format

The biggest advantage of EML is cross-platform compatibility. Thunderbird, Apple Mail, Windows Live Mail, and a host of Linux clients all handle EML natively. You can save an EML on a Windows machine, transfer it to a Mac, and open it in Apple Mail without any conversion step. Try that with an MSG file and you'll hit a wall.

EML is also the better choice for long-term archiving. Because it's an open, non-proprietary standard, you're not dependent on a single vendor's continued support. I've seen organizations pull EML files from archives that were over a decade old and open them without issue. The same can't always be said for proprietary formats.

For email migration, EML is the format that gets you out of vendor lock-in. If you're moving from Outlook to Gmail or Thunderbird, exporting your messages as EML gives you a portable archive that your new client can import directly.

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What Is the MSG File Format? The Proprietary Powerhouse of Outlook

MSG is Microsoft's proprietary format for storing Outlook items. Built on the OLE Compound File structure, MSG files are binary—not text—and they preserve a wealth of Outlook-specific metadata that EML simply can't represent.

If you work exclusively within the Microsoft ecosystem, MSG is a powerhouse. It maintains message flags, categories, custom properties, RTF formatting, and even meeting requests and voting buttons. But that power comes at a cost: MSG files are essentially locked to Outlook and Exchange.

Features and Structure of MSG Files

The OLE Compound File structure is like a mini file system inside a single file. It organizes data into hierarchical streams, allowing Outlook to store multiple types of data within one MSG file. This is why MSG files can contain embedded objects, custom MAPI properties, and rich formatting that goes beyond what standard email headers can hold.

Here's what MSG preserves that EML typically can't:

• Message flags: Read/unread status, follow-up markers, importance levels
• Categories: Color categories and custom tags for organization
• Custom properties: Extended MAPI fields specific to your organization
• RTF formatting: Rich Text Format that goes beyond basic HTML
• Delivery receipts: Read confirmations and delivery tracking
• Conversation tracking: Thread management data

The binary structure stores each component in separate streams. Headers occupy one stream, body content another, and attachments exist as embedded sub-files. This allows Outlook to access specific parts of the message without loading the entire file.

MSG also preserves Exchange Server properties that generic formats can't represent—server-side rules, retention policies, and compliance tags that matter for enterprise management.

Benefits and Advantages of the MSG Format

Within Microsoft environments, MSG delivers maximum fidelity. When both parties use Outlook, MSG files transfer perfectly with zero data loss. Custom formatting, embedded objects, and Outlook-specific features survive intact.

Enterprise workflows benefit from Exchange integration. MSG files maintain server-side properties, retention tags, and compliance markers that regulated industries depend on. eDiscovery tools designed for Microsoft environments parse MSG natively, which is why legal teams in Outlook-centric organizations often prefer this format.

MSG also supports advanced Outlook features like voting buttons, meeting requests, and task assignments. These objects require MSG format to function properly when shared outside a live Exchange connection.

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EML vs MSG: A Head-to-Head Comparison for Real-World Use

Now let's get practical. Here's how the two formats stack up across the criteria that actually matter in day-to-day email management.

Compatibility Across Email Clients and Platforms

Cross-platform considerations matter too. EML files work identically on Windows, macOS, and Linux. You can save an EML on a Windows machine, transfer it to a Mac, and open it in Apple Mail without conversion. MSG shows a strong Windows bias—while Outlook runs on macOS, other platform-native clients can't open MSG directly. Linux users face significant barriers, often requiring third-party conversion tools.

One thing I've noticed in recent years: the new Outlook for Windows changed its drag-and-drop behavior. Classic Outlook saves messages as MSG when you drag them to the file system, but the new Outlook saves them as EML. That's a subtle shift that's caught many users off guard.

File Size, Attachments, and Metadata Preservation

When it comes to metadata, the gap is significant. EML preserves standard email headers—sender, recipient, subject, date—but that's about it. MSG preserves everything Outlook knows about the message: flags, categories, custom properties, RTF formatting, and more. If you need that metadata for compliance or internal audits, MSG is the only choice.

Security Considerations for Email Files

Both formats carry security risks through malicious attachments or embedded content. But the way they're scanned differs.

EML's text-based structure allows straightforward antivirus scanning. Security tools can parse MIME boundaries and extract attachments without needing to understand a complex binary format. That transparency is a security advantage.

MSG files require OLE-aware security scanning. The binary structure can obscure malicious payloads within compound streams, making them harder to inspect. This doesn't mean MSG files are inherently less safe—it just means the scanning tools need to be more sophisticated.

Here are some best practices I recommend for handling both formats:

• Scan all email files with updated antivirus software before opening
• Use sandboxed environments for files from unknown sources
• Disable automatic macro execution in email clients
• Verify sender authenticity before opening attachments
• Keep your email client updated to patch known vulnerabilities

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How to Convert EML to MSG: A Step-by-Step Guide

Conversion becomes necessary when you're migrating between email systems or need to share messages across platforms. Here are the three main approaches, each with its own trade-offs.

Method 1: Using Microsoft Outlook (Manual Conversion)

The most straightforward method is to import the EML file into Outlook and then save it as MSG. Here's how:

1. Open Outlook.
2. Drag the EML file into any folder (or use File > Open & Export > Import/Export).
3. The message appears in Outlook with its content intact.
4. Select the message and go to File > Save As.
5. Choose "Outlook Message Format (*.msg)" from the file type dropdown.

A word of caution: this process can lose some metadata. Outlook categories, custom properties, and RTF formatting from the original EML won't magically appear—Outlook can only preserve what's already in the file. And the "Save As" behavior varies by Outlook version. Classic Outlook for Windows defaults to MSG, while the new Outlook may save as EML.

Method 2: Using Dedicated Conversion Tools (Batch Processing)

If you're converting more than a handful of files, manual conversion becomes tedious. Dedicated tools like CoolUtils Total Mail Converter or MailsWare EML Converter handle batch processing efficiently.

These tools typically offer:

• Bulk conversion of hundreds or thousands of files
• Preservation of folder structure
• Options for metadata preservation
• Command-line support for automation

The trade-off is cost—most reliable tools are paid—and the learning curve. But if you're converting files regularly, the time savings are worth it.

Method 3: Free Online Converters (Quick Solutions)

For occasional conversions, online services like Zamzar or Convertio work fine. The process is simple: upload your EML file, select MSG as the output format, and download the result.

Here's the catch: privacy. Uploading sensitive emails to a third-party service is risky. I've seen organizations inadvertently expose confidential data this way. If you're converting anything beyond test messages, use a desktop tool instead.

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Which Is Better for Email Backup and Archiving?

This is where the choice gets strategic. Your backup format determines how accessible your emails will be years from now.

Archiving for Compliance and Legal Discovery

For legal holds and compliance, EML's open standard is often preferred. Courts and regulatory bodies accept EML because it's a transparent, verifiable format. You can open an EML file in any text editor and verify its contents without specialized software. That transparency builds trust in the evidence.

MSG, on the other hand, excels at preserving Outlook-specific metadata for internal audits. If your organization needs to prove that a message was flagged as important, or that a particular category was applied, MSG retains that information while EML doesn't.

Regulatory requirements can tip the scales. For example, SEC regulations in the United States require broker-dealers to preserve electronic communications in a format that's "non-rewriteable, non-erasable." Both formats can satisfy this, but EML's open standard makes it easier to demonstrate compliance.

A Decision Framework for Your Specific Needs

Here's a simple way to think about it:

Choose EML if:

• You need cross-platform email client compatibility
• You're sharing messages with external organizations
• You're archiving for long-term compliance
• You work in a mixed-OS environment (Windows/macOS/Linux)
• You want flexibility for future platform migrations

Choose MSG if:

• You work exclusively in Microsoft Outlook/Exchange environments
• You need full Outlook metadata preservation
• Your organization is Windows-centric
• You're using eDiscovery tools designed for Microsoft
• You need advanced Outlook features like voting buttons or meeting requests

If you're still unsure, try this: convert a test email to both formats and open them in your primary email client. See which one preserves the information you actually need.

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Common Issues and Troubleshooting for EML and MSG Files

Even with the right format, problems arise. Here are the most common issues I've encountered and how to fix them.

Why Can't I Open an MSG File Outside of Outlook?

This is the most frequent complaint. MSG is proprietary, so most non-Microsoft clients can't open it natively. Thunderbird requires an extension like "ImportExportTools" to handle MSG files. Apple Mail has limited support.

Your options:

• Use a dedicated MSG viewer (many free options exist for Windows, macOS, and Linux)
• Convert the MSG to EML using a conversion tool
• Open it in Outlook and re-save as EML

If you're having trouble opening an EML file in Outlook, the process is simpler: just drag the EML file into an Outlook folder, and it will import automatically.

Troubleshooting Conversion and Formatting Issues

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Frequently Asked Questions

Is it better to save as EML or MSG?

It depends on your workflow. Save as EML if you need cross-platform sharing or long-term archiving. Save as MSG if you're working exclusively in Outlook and need full metadata fidelity. In my experience, most users benefit from EML for external communication and MSG for internal Outlook workflows.

What is the main difference between EML and MSG files?

EML is an open, text-based MIME standard that any email client can read. MSG is a proprietary, binary format from Microsoft that preserves Outlook-specific metadata like flags, categories, and custom properties. The core difference is openness versus feature completeness.

How do I convert an EML file to MSG format?

You have three options: import the EML into Outlook and save as MSG (manual, good for occasional conversions), use a dedicated conversion tool (best for batch processing), or use a free online converter (quick but raises privacy concerns for sensitive emails).

Can MSG files be opened by email clients other than Outlook?

Not natively. Thunderbird requires extensions, and Apple Mail has limited support. The most reliable way to open an MSG file outside Outlook is to convert it to EML first.

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Conclusion

The choice between EML and MSG comes down to your specific workflow and environment. EML offers universal compatibility, transparency, and long-term archival stability through open standards. MSG delivers maximum feature preservation within Microsoft Outlook environments, making it the right choice for Outlook-centric organizations that need full metadata fidelity.

Here's my advice: evaluate your current email workflow and identify one area where you can apply this decision framework. If you're still unsure, convert a test email to both formats and compare the results in your primary email client. The right choice will become obvious when you see which format preserves what you actually need.