Learn about the EML file format — the universal standard for email messages. Understand what EML files contain, which apps open them, and how to convert them.

What Is an EML File?

An EML file is a single email message saved in the MIME (Multipurpose Internet Mail Extensions) format, following the RFC 2822 and RFC 5322 standards. The .eml extension stands for “electronic mail,” and each file represents exactly one email — including its headers, body text, HTML content, and all attachments.

EML is the closest thing to a universal email format. Unlike PST (Outlook-only) or MBOX (archive-oriented), EML files are recognized by virtually every email client: Thunderbird, Apple Mail, Windows Mail, eM Client, and many more. They are also human-readable at their core — you can open an EML file in a plain text editor and see the raw message structure.

What Is Inside an EML File?

An EML file is structured as a MIME message with clearly defined sections:

Headers — metadata about the message: From, To, CC, BCC, Subject, Date, Message-ID, MIME-Version, and Content-Type. These are plain text key-value pairs at the top of the file.
Body (plain text) — the text/plain version of the message content.
Body (HTML) — the text/html version with formatting, links, and embedded styles.
Attachments — files attached to the email, encoded in Base64 within the MIME structure. Each attachment has its own Content-Type and Content-Disposition headers.
Inline images — images embedded in the HTML body are included as MIME parts with Content-ID references.

A typical EML file looks something like this when opened in a text editor:

From: sender@example.com
To: recipient@example.com
Subject: Meeting notes
Date: Wed, 26 Mar 2026 10:30:00 +0200
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="boundary123"

Which Applications Open EML Files?

One of EML’s greatest strengths is broad compatibility:

Mozilla Thunderbird — drag and drop or File, then Open Saved Message.
Apple Mail — double-click the EML file in Finder.
Windows Mail / Mail app — double-click opens the message.
Microsoft Outlook — drag the EML file into an Outlook folder (not native open, but functional).
eM Client — full native support.
Text editors — any text editor can display the raw MIME source.
Web browsers — some browsers render the HTML portion if you open the file directly.

How Are EML Files Created?

Drag from email client — most email clients let you drag a message from the message list to the desktop or a folder, creating an EML file.
Save as — Thunderbird and other clients offer a “Save As” option that writes the message as an EML file.
Export/conversion — tools like MailtoPst can extract messages from PST, OST, or MBOX archives and save each one as an EML file. See PST to EML.
Server-side — many email servers can export messages in EML format via IMAP FETCH commands.

When to Use EML Format

EML files are ideal for several use cases:

Cross-platform sharing — send an email to someone who does not use the same email client, and they can open the EML on any system.
Archival — store important emails as individual files in a document management system or cloud storage.
Legal and compliance — courts and regulators accept EML as a standard format for email evidence, since it preserves original headers and metadata.
Migration — when moving between email clients, converting to EML provides an intermediary format that any target client accepts.

How to Convert EML Files

If you need to go from EML to another format, MailtoPst handles common conversions:

EML to PST — combine multiple EML files into a single Outlook archive.
EML to MBOX — merge EML files into an MBOX archive for Thunderbird.

All conversions run securely in the cloud — files are encrypted via TLS, processed on EU servers, and auto-deleted afterward. Try free — no credit card required.

Frequently Asked Questions

Is EML the same as MSG?

No. EML follows the open MIME/RFC 2822 standard and is supported by most email clients. MSG is Microsoft’s proprietary format used by Outlook. MSG files contain additional Outlook-specific properties. See our blog on MSG vs EML for details.

Can I open an EML file without an email client?

Yes. EML files are plain text at their core. Open one in Notepad, TextEdit, or VS Code to see the raw MIME structure. For a rendered view with proper formatting, use an email client or upload to MailtoPst.

Are EML files safe?

The EML file itself is a text document and is not executable. However, it can contain attachments that may be malicious. Always scan attachments with antivirus software, just as you would with any email.

How large can an EML file be?

There is no technical limit in the format specification. In practice, EML file size depends on the message content and attachments. A plain text email might be a few kilobytes, while an email with large attachments could be hundreds of megabytes.

Can I batch-convert EML files to PST?

Yes. Upload a ZIP containing all your EML files to mailtopst.com/convert/eml-to-pst and receive a single PST archive. See How to convert EML to PST.

Upload your file now to convert or explore your EML messages.

What Is an EML File?