What Is an .eml File?

An .eml file is a single email saved as a standalone file. It holds everything the message contained when it arrived — the sender, recipients, subject, date, the message body, and any attachments — stored as plain text in a format that almost every email program can read.

If someone sent you one, or you found one in a folder and your computer doesn't know what to do with it, it's just an email sitting outside your inbox.

You can open one right now in your browser — drop it into the .eml viewer. Nothing is uploaded; the file is read on your own device.

Where .eml files come from

You end up with .eml files in a handful of ordinary ways:

• Saving a message out of your mail program. Outlook, Thunderbird, Apple Mail, and Windows Mail all export individual messages this way.
• Downloading a message from webmail. In Gmail, opening a message and choosing Show original → Download original produces one.
• Receiving a forwarded email as an attachment. When someone forwards a message "as attachment" rather than inline, what lands in your inbox is an .eml.
• Exports from archiving, e-discovery, or backup tools. Legal holds and mailbox exports frequently produce large folders of them.
• Migrating between email providers. Bulk exports often arrive as thousands of .eml files in a zip.

That last group is why so many people meet this format for the first time under deadline pressure — a mailbox export lands, and none of it opens by double-clicking.

What's actually inside one

Open an .eml in a text editor and you'll see something readable. That's not a coincidence: the format is defined by RFC 5322, the same internet standard that governs how email moves between servers. An .eml file is essentially a transcript of the message as it travelled.

The structure is simple. Headers come first, one per line:

From: Dana Reed <dana@example.com>
To: accounts@yourcompany.com
Subject: Invoice 4417
Date: Tue, 29 Jul 2026 09:14:02 -0400
Content-Type: multipart/mixed; boundary="XYZ"

Then a blank line, then the body.

Most real emails carry more than one version of the body — a plain-text version and an HTML version — so the recipient's program can display whichever it handles better. Those, plus any attachments, are separated by boundary markers and stacked inside the same file. This layering is described by the MIME standard.

Attachments live inside the file too, converted into text using base64 encoding. That's why a 2 MB photo makes the .eml roughly 2.7 MB — base64 adds about a third to the size. It's also why an attachment looks like meaningless character soup when you view the raw file, and why you need a program that understands the format to get the original file back out.

Above the visible headers sits something most people never look at: the Received chain, a stack of entries added by each mail server the message passed through, newest first. It records the route and timing of the delivery, which is what makes .eml files valuable as evidence in fraud, harassment, and dispute investigations. A forwarded screenshot proves nothing. The original file carries the routing history.

What .eml is not

It isn't a Microsoft format. The extension came from Outlook Express, but the underlying format is an open standard. Thunderbird, Apple Mail, Windows Mail, and most other clients read it.

It isn't a mailbox. One .eml is one message. Formats that hold entire folders are different: .mbox (used by Thunderbird and Apple Mail) and .pst (Outlook's archive format) each contain many messages.

It isn't .msg. That's Outlook's proprietary format for a single message — same job, different, non-portable format. The differences are worth knowing if you're deciding how to archive something.

It isn't executable. An .eml file cannot run on its own. It can, however, contain a dangerous attachment, so the usual caution applies to anything you extract from one.

Can you read an .eml file without an email program?

Yes. Because it's plain text underneath, a text editor will show you the headers and any plain-text body. That's often enough to answer "who sent this and when."

What a text editor won't do is render the formatting, decode the attachments, or make sense of a message that only has an HTML body — you'll see raw markup or base64 blocks instead. For that you need something that understands the structure.

This browser-based viewer does it without installing anything: it parses the file locally, shows the message as it was sent, lists the attachments as downloadable files, and can save the whole thing as a PDF. The file never leaves your device.

Is it safe to open an .eml file?

Opening one to read it is low-risk in a viewer that doesn't execute anything. Two real cautions:

Attachments inside it. An .eml can carry any file type, including malicious ones. Extracting and running an attachment carries exactly the same risk as it would have in your inbox.

Remote images. Marketing and phishing emails commonly embed an invisible image hosted on the sender's server. Loading it reports back that the message was opened, when, and roughly from where. Good viewers block these by default. This matters more than people expect if you're reviewing a suspicious email — you don't necessarily want the sender to know you've read it.

Common questions

Are .eml files readable? Yes — in a text editor you'll see headers and plain-text content. To see formatting and attachments properly, use an email program or a dedicated viewer.

Can I open an .eml file on my phone? Sometimes. iOS previews them in Mail; Android support varies by device and app. A browser-based viewer works on any phone, which is usually the simplest route.

How do I convert one to PDF? Open it in a viewer and print to PDF, which preserves the text as selectable rather than a screenshot. Here's the full walkthrough.

Why is my .eml file so large? Attachments are base64-encoded inside the file, which inflates them by roughly 33%.

What program opens .eml files by default? On Windows, usually Outlook or Windows Mail if installed. On macOS, Apple Mail. If nothing is associated, Windows shows a "how do you want to open this file" prompt — here's what to pick.

Need to open one now? Use the free .eml viewer — it runs entirely in your browser.