EML vs MSG: What They Are and How They Differ

An .eml file is a single email message stored exactly as it travelled the internet: plain text, in MIME format, readable in any text editor. A .msg file is a single email message stored the way Microsoft Outlook keeps it internally: a binary container that shows you nothing without Outlook or a tool built to decode it.

Same message, two wrappers. The wrapper decides whether the file still opens in ten years without the vendor that made it.

What is actually inside an .eml file

Open one in TextEdit or Notepad and you are reading the message itself. Header lines at the top — From, To, Subject, Date, Message-ID and a stack of technical ones — then a blank line, then the body. An HTML version and a PDF attachment are MIME parts inside that same file, the binary encoded as text so it survives the trip.

That is the whole thing. EML is not a file format somebody designed; it is the message format defined by RFC 5322 and the MIME standards, written to disk unchanged. The extension is a convention rather than a container, which is why Gmail’s “Show original” gives you those same bytes and renaming the download to .eml makes it open like any other message file.

What is actually inside a .msg file

Bytes you cannot read. MSG is Outlook’s own format, built on the Compound Document Structure — the OLE2 container that old .doc and .xls files used. Inside, the message is broken into named streams: one for the subject, one for each version of the body, one per recipient, one per attachment.

It is proprietary but not secret: Microsoft publishes the specification, which is why third-party libraries can read MSG at all. The practical situation is unchanged, though. Nothing that ships with macOS opens a .msg, nothing that ships with Windows does either, and neither does Thunderbird.

The comparison, without the marketing

	EML	MSG
Defined by	RFC 5322 and MIME, open standards	Microsoft, one vendor
On disk	Plain text	Binary OLE2 container
Readable in a text editor	Yes	No
Opens on	Any operating system, most mail clients	Outlook on Windows, or a converter
Holds	Exactly what SMTP delivered	The message plus Outlook’s own properties
Can hold non-mail items	No	Yes — appointments, contacts, tasks

The row that matters most is the second to last. An EML holds the message. An MSG holds the message plus properties that never travelled over the network: categories, follow-up flags, read and unread state, the internal Exchange address of a colleague instead of their public one, whether a task was completed. Convert an MSG to EML and those are dropped, because the standard has nowhere to put them.

That is the one honest argument for keeping MSG files. When what needs preserving is Outlook’s own bookkeeping — the category tags a legal team applied, an appointment saved as a file — the MSG is the record and a conversion loses part of it. For the email itself the argument does not apply: the message survives intact.

Which one you will actually end up with

Drag a message out of classic Outlook for Windows onto the desktop and you get .msg. That one habit accounts for most of the MSG files in circulation, and it is rarely a decision anyone made — it is just what Outlook does.

Nearly everything else produces .eml: Thunderbird’s Save As, a message dragged from Apple Mail to the Finder, Outlook on the web, ticketing systems, office scanners, mail servers, archiving appliances.

Apple Mail is the exception worth naming, because it confuses people twice. What it stores is EMLX: one message per file, with the byte length on the first line and an XML property list of Apple’s metadata at the end. Strip those two and an EMLX is an EML. What it exports is neither — a folder called Inbox.mbox containing a file called mbox, which is standard MBOX.

Opening each one when you do not have Outlook

EML is easy. On a Mac, double-clicking opens it in Mail as a single message window; Mail displays it, it does not import it into your mailboxes. On Windows 10 and 11 the file goes to whatever mail app is registered, and if none is, opening it in Notepad shows the raw message — enough to read the headers and see who really sent it. Mbox Viewer opens .eml files directly on both systems, alongside MBOX.

MSG is not easy, and it is worth saying plainly: Mbox Viewer does not open .msg files. You need Outlook, a dedicated MSG viewer, or a converter. If you still have access to the mailbox the file came from, the cheapest route by far is to open that message in Outlook on the web and download it — you get an EML and the problem stops existing.

How both of them relate to MBOX

An MBOX is many messages in one file, each preceded by a line beginning From . An EML is one of those messages on its own. That is the entire relationship, and it is why converting between them is bookkeeping rather than engineering: split an MBOX at the From lines and you have a folder of EMLs; put a From line in front of each EML, concatenate them, and you have an MBOX. Mbox Viewer exports selected messages to EML without paying for anything, and the conversion guide covers both directions.

MSG has no such shortcut. It has to be decoded first, which is why every route out of Outlook runs through a converter — the same detour a PST file needs, and the reason it has a guide of its own.

One caution before you turn an archive into individual files: a folder holding 200,000 EMLs is miserable. Finder and File Explorer crawl, backups take hours, and searching across them is slower than searching one big file. For a handful of messages, EML. For an archive, MBOX.

If you get to choose

Choose EML, for the same reasons MBOX beats PST: it is the format your mail already exists in while it is in flight, and it stays readable in a text editor long after every program that could open it is gone.

Keep MSG only when Outlook’s own properties are the point. And if the honest answer is “thousands of messages”, pick neither: put them in an MBOX and keep one file instead of a directory nobody can face.