EML vs MSG File Format: Key Differences & Comparison Guide

An .eml file is one email message stored as plain text, written by almost every mail program — Apple Mail, Thunderbird, Gmail exports and Windows Mail. A .msg file is one message stored in Outlook’s own binary container. They hold the same kind of thing in completely different formats, which is why a .eml opens in any text editor and a .msg looks like gibberish in one.

That single difference — text versus binary container — is behind almost every practical question people have about the two formats: which one a program produces, why one won’t open, and how to read either without the software that made it. Here is what actually distinguishes them, tested against real files.

The short version

• .eml — plain RFC 5322 text: header lines, a blank line, then the body. Universal. Opens in Notepad, though it looks messy.
• .msg — a Microsoft Compound File Binary container, the same structure a Windows Installer .msi uses. Outlook for Windows writes it. Nothing else reads it without a dedicated parser.
• Both hold exactly one message. Neither is a mailbox — that is .pst, .ost or .mbox, which hold thousands.

What each format actually is

A .eml is the message exactly as it travelled between mail servers. Open one in a text editor and you can read the From:, To:, Subject: and Date: lines directly. The catch is that the parts which contain anything beyond plain ASCII are encoded: a subject with an accent arrives as =?UTF-8?B?…?=, and attachments are Base64 — long blocks of seemingly random letters. It is readable in principle and unreadable in practice, which is the whole reason viewers exist.

A .msg is not text at all. It is a miniature file system inside a single file, with each part of the message — subject, body, every attachment — stored as a separate named stream. The names look like __substg1.0_0037001F, where the digits are a Microsoft property code. This is why a .msg shows as binary noise in a text editor: there is no plain text to find. It needs software that understands the container.

Which program makes which

You got the file from…	It is almost certainly…
Dragging a message out of Outlook for Windows	.msg
Outlook on Mac, or Outlook on the web	.eml
Apple Mail (drag to desktop)	.eml
Thunderbird “Save As”	.eml
A Gmail or Google Workspace export	.eml
A system that “saved the email” on Windows	usually .msg

The rule of thumb: .msg means Outlook for Windows was involved. Everything else produces .eml, because .eml is the format email itself uses.

How to tell which one you have

Open the file in a plain text editor — Notepad on Windows, TextEdit on Mac.

1. If the first lines are readable words like Received:, From: and Subject:, it is a .eml.
2. If it opens as a wall of binary characters starting with a few odd symbols, it is a .msg. (The first eight bytes are always the same signature, D0 CF 11 E0, though you will not see that in a text editor.)
3. If the file is hundreds of megabytes, it is neither — it is a mailbox (.pst or .mbox), which holds many messages.

Which should you keep?

If you…	Prefer
Need the file to open on any computer, now or in ten years	.eml
Work entirely inside Outlook for Windows and value exact fidelity	.msg
Are archiving for legal or compliance reasons	.eml (open, documented, future-proof)
Need to move a message between different mail programs	.eml

For long-term storage, .eml is the safer choice precisely because it is plain text and its format is a published standard. A .msg depends on Microsoft’s container format and reads best in Outlook — fine while you are in that world, less so as an archive meant to outlast it.

Common problems, and what causes them

• “The .msg file won’t open on my Mac.” Expected — macOS has no built-in support for Outlook’s container. The message is intact; it just needs a reader that understands the format.
• “The subject is full of =?UTF-8?B? gibberish.” That is a .eml with an encoded-word subject. A proper viewer decodes it back to the real characters; a text editor shows it raw.
• “The attachment saved as a text file full of random letters.” That is Base64 — the raw encoding of a .eml attachment. It has to be decoded back to bytes, which a viewer does and a copy-paste does not.
• “The first line of the subject is missing.” Usually a byte-order mark or a folded header that a naive parser mishandled. Reader software that expects these keeps the whole subject.

Reading either one without the mail client

You do not need Outlook, or any account, to read either format. EasyExtract has a viewer for each, and both run entirely in your browser — the message is opened on your own device and never uploaded, which matters because a saved email is real correspondence.

• Outlook MSG viewer — opens a .msg, shows the sender, recipients, date and body, and saves every attachment at full size.
• EML viewer — opens a .eml, decodes the headers and Base64 attachments, and shows the body as text.

Both show the message body as text rather than rendering its HTML, so tracking pixels never load and links are never live — the safe way to read a message you are not sure about.

Frequently asked questions

Can I convert a .msg to a .eml?

Yes, but it is a genuine conversion between two different containers, not a rename. Outlook for Windows can save a message as .eml directly; on other systems, a viewer that reads the .msg and re-exports the message is the practical route. Simply changing the file extension does not work — the bytes inside are still Outlook’s binary format.

Is one format more secure than the other?

No. Both are just containers for the same message, and neither encrypts anything by itself. What matters for safety is how you open the file: a viewer that shows the body as text rather than rendering its HTML will not load tracking pixels or activate links, regardless of whether the source was .eml or .msg.

Why does Outlook use .msg instead of the standard .eml?

The .msg format predates the web-era standardisation of email files and stores Outlook-specific properties — categories, flags, internal fields — that plain RFC 5322 has no place for. It is higher fidelity inside Outlook, at the cost of portability everywhere else.

Which opens on the most devices?

.eml, by a wide margin. Because it is plain text following a public standard, essentially every mail program on every operating system can read it. A .msg reliably opens only in Outlook for Windows, or in software written specifically to parse its container.

Sources

• RFC 5322: Internet Message Format — the standard a .eml file follows.
• [MS-CFB]: Compound File Binary File Format — the container a .msg file uses.
• [MS-OXMSG]: Outlook Item (.msg) File Format — how a message is laid out inside that container.