An EML file is a portable, plain-text email message file that follows the MIME standard, so Thunderbird, Apple Mail, many email viewers and browser-based tools can open it. An MSG file is a proprietary binary format created by Microsoft Outlook. Choose EML when you want to share, preview, archive or inspect an email outside Outlook. Choose MSG only when the recipient needs native Outlook behavior and metadata.
Want to see an EML file right now? Import it into the free HTML Email Tester and preview it in your browser. Nothing to install, no signup.
What Is an EML File?
An EML file is a saved copy of a single email message stored as plain text. It follows the Internet Message Format (RFC 5322, formerly RFC 822) and uses MIME (Multipurpose Internet Mail Extensions) to package everything in one file.
Open an EML file in a text editor and you can read the email headers directly: From:, To:, Subject:, Date:, Message-ID: and Content-Type:. MIME boundaries split the message into parts, such as the plain-text body, the HTML body, inline images and file attachments. Inline images are normally linked from the HTML through cid: references that match a Content-ID header. A .mime file is often the same kind of message saved with a different extension.
Because EML is an open, human-readable standard, it is the most portable email file format. That makes it the usual choice for email archiving, HTML email testing, debugging and cross-platform sharing.
What Is an MSG File?
An MSG file is Microsoft Outlook's native message format. Instead of readable text, it is a binary file built on a compound storage structure (OLE). It stores the message body, recipients, attachments and Outlook-specific MAPI properties, such as categories, flags and other internal fields.
If you open an MSG file in Notepad you will see unreadable characters. That is normal, not corruption. You need Outlook or a parser that understands the Outlook structure to read it.
MSG works well inside Microsoft-first workplaces, where preserving Outlook metadata matters. Outside Outlook, support is limited.
EML vs MSG: Side-by-Side Comparison
| Feature | EML | MSG |
|---|---|---|
| File type | Plain-text MIME message (RFC 5322 style) | Binary Outlook message container |
| Created for | Many email clients and tools | Microsoft Outlook |
| Readable in a text editor | Yes, headers and body are visible | No, it shows binary data |
| Stores | Headers, plain-text and HTML parts, inline images, attachments | Body, recipients, attachments, Outlook metadata and MAPI properties |
| Portability | High, cross-platform | Lower, many non-Outlook tools can't open it |
| Opens in | Thunderbird, Apple Mail, Outlook (usually), browser-based EML viewers | Outlook desktop, specialized MSG viewers |
| Best for | HTML email QA, sharing proofs, newsletters, email archives, debugging | Outlook-to-Outlook handoff, Outlook-centered archives |
Both formats store one email message, which is why people assume they are interchangeable. They are not. Renaming a file from .msg to .eml does not convert it. The contents stay in Outlook's binary structure and still won't open in tools that expect MIME text.
Which Email Format Should You Save: EML or MSG?
Save as EML when:
- The recipient may not use Outlook
- You want to preview the HTML email in a browser
- You need to inspect email headers, links or source code
- You want a portable, future-proof copy for email archiving
- You are sending proofs to a design, QA or marketing team
Save as MSG when:
- The recipient will open the message in Outlook desktop
- You must preserve Outlook-only metadata
- A compliance, legal or eDiscovery workflow specifically requires MSG
Menu labels differ between Outlook versions, so always check the file extension after saving. What matters is whether the file ends in .eml, .mime or .msg.
How to Open an EML File (4 Methods)
Method 1: Preview the EML File in Your Browser
This is the quickest way to open an EML file when you need more than a basic read. The HTML Email Tester runs entirely in your browser, so there is nothing to install.
- Open the tool and import your .eml or .mime file. You can also paste HTML or import .html and .htm files.
- Pick a client-style preview, then switch between desktop and mobile views and light and dark themes.
- Click Check Email to review the source, plain text, headers, HTML compatibility, links and the QA report.
- Download the HTML as email-template.html if you want to keep editing.
Editing, preview and checks run locally in your browser, and nothing is hosted on ToolsViewer servers. You can also format HTML, inline CSS, minify code and test merge tags like {{variable}} by adding sample JSON in the Payload option.
After editing, use HTML Compare to confirm only the intended changes were made. To read source in more depth, see our guide to the best free HTML viewers in 2026.
Method 2: Open the EML File in an Email Client
Many desktop email clients, including Apple Mail and Thunderbird, open EML files directly. Outlook desktop can usually open them as well. Double-click the file or drag it into the app.
This shows the message the way a mail client displays it. It is less convenient for inspecting raw headers and source than a dedicated QA tool, and it won't let you test mobile or dark mode layouts.
Method 3: Read the EML File in a Text Editor
Because EML is plain text, any text editor will show the raw MIME structure. This is useful for email forensics, spam analysis and troubleshooting delivery problems, since you can inspect the full header trail, authentication results and encoded parts. The HTML body may be encoded as quoted-printable or base64, so it can look messy until it is decoded.
Method 4: Use an Online EML Viewer
An online EML viewer lets you open an email message file without any desktop software. Choose a tool that processes the file locally in your browser, especially for sensitive messages, so your email content is not uploaded to a third-party server.
How to Open an MSG File
MSG files open natively in Outlook desktop. Double-click the file and Outlook shows the message, attachments and metadata.
Without Outlook, you need an MSG viewer or parser that understands Outlook's storage format, and many general-purpose tools cannot read it. The HTML Email Tester accepts .eml, .mime, .html and .htm files but not .msg, so convert the MSG file to EML first.
How to Convert MSG to EML
Convert MSG to EML when a non-Outlook user needs to view the message, when an HTML email team needs the body for QA, or when you want a portable archive.
- Don't just rename the file. A true conversion reads the Outlook container and writes a MIME message.
- Convert in Outlook or with a dedicated converter. Use a tool you trust, particularly for confidential email.
- Verify the result. Confirm the HTML body, inline images and attachments survived.
- Keep the original MSG if the message has compliance, legal or record-keeping value.
If the original relied on Outlook-only features, some metadata may not map cleanly into EML. Treat the converted file as a working copy, not a perfect replica.
Recommended Workflow for HTML Email Teams
For email marketers, developers and QA teams, portability beats Outlook-specific metadata. A reliable process looks like this:
- Ask teammates to send saved campaign proofs as EML or MIME files.
- Import the file into the HTML Email Tester and review the rendered body.
- Check headers, links, source code and the desktop, mobile and dark mode previews.
- Fix problems in the HTML and download the updated template. If you author in MJML, fix it in the MJML Email Builder instead. New to it? Read What Is MJML?
- Send a real test before launch. You can test email with Mailpit using your own HTTPS endpoint.
- Need a visual record for sign-off? Capture the rendered HTML as a PDF, PNG or JPG. These are snapshots of the visible body, not a full mail archive.
- Keep MSG only for Outlook-native handoff or archive rules that require it.
Troubleshooting Common EML and MSG Problems
- The file shows unreadable characters in a text editor. That is expected for MSG, which is binary. Open it in Outlook or convert it to EML first.
- The email tester won't accept my file. Check the extension. The tool imports .eml, .mime, .html and .htm, but not .msg.
- Renaming MSG to EML didn't work. Renaming only changes the extension. The content is still Outlook's binary format and needs a real conversion.
- Images are missing in the preview. Inline images rely on cid: references that match related MIME parts. If those parts were not saved in the file, the images cannot load.
- Attachments or metadata are missing after conversion. Some Outlook-only details don't map into EML. Compare the converted file with the original MSG.
- The preview looks different from my inbox. Previews are client-style simulations, not the real Gmail or Outlook rendering engines. Always send a live test through your email service provider before a major campaign.
Why Do Two Email File Formats Exist?
The formats come from different goals. EML follows the MIME standard, which allows different mail systems to exchange messages. MSG belongs to Outlook's own storage model and can carry Outlook-specific context, which is why it works well in Microsoft-first offices and poorly elsewhere.
For HTML email work, portability usually matters more than Outlook metadata. EML lets a teammate on any operating system preview a proof, inspect the source and check the layout.
FAQ
Is EML better than MSG?
Can I open an EML file without Outlook?
Can I open an MSG file without Outlook?
Is an EML file safe to open?
Does converting MSG to EML lose data?
What is a .mime file?
Conclusion
EML and MSG both save an email message, but they serve different jobs. Pick EML for portability, QA, sharing and long-term email archiving. Pick MSG when your workflow depends on Outlook. If you receive an MSG file and need to inspect it elsewhere, convert it to EML, then import it into the free HTML Email Tester to preview the layout, check links and review headers, all in your browser.