EML vs MSG Files: Which Email Format Should You Export?

The EML vs MSG file format question comes up the moment you try to save an email as a file and see two options that look interchangeable but are not. An EML file is a raw email message in the format the internet itself uses — plain, standardised text that almost any program can read. An MSG file is Microsoft's Outlook Item format, a binary file that stores the same message plus the extra properties Outlook keeps about it. They are not two flavours of one thing; they are built for different destinations.

Here is the short version, so you can act before reading the rest. If you are archiving mail for the long term, moving it between different programs, or handing it to someone who does not use Outlook, choose EML. If you need to keep Outlook-specific details — colour categories, flags, follow-up state, custom fields — or you are round-tripping a message inside an Outlook-only workflow, choose MSG. The rest of this guide explains why, and the cases where the easy answer is wrong.

The mistake worth avoiding is treating the two as identical. Export the wrong one and you either drop metadata you needed or produce a file the receiving system cannot open. So the useful question is never "which is better" in the abstract — it is "better for where this file is going." That is what this comparison is organised around.

What is an EML file? What is an MSG file?#

An EML file is a single email message saved in the exact form email travels in on the wire. That form is defined by RFC 5322, the IETF's Internet Message Format standard, which describes a message as header fields (From, To, Subject, Date, Message-ID and the rest) followed by an optional body, written in plain US-ASCII text. Because it is text following a public standard, an .eml file opens in nearly any mail client — Apple Mail, Thunderbird, Outlook itself — and you can even read its source in a plain text editor.

Attachments, HTML formatting and non-ASCII characters ride inside that same text file through MIME (the Multipurpose Internet Mail Extensions, RFC 2045 onward), which encodes them into the message body. That is why a single, correctly built .eml file is self-contained: the body, the HTML version, inline images and file attachments are all packed into one message.

An MSG file is Microsoft's Outlook Item file format, documented in the open specification MS-OXMSG. Per Microsoft's own description, it is "a syntax for storing a Message object, such as an e-mail, an appointment, a contact, a task, and so on, in a file," and "any properties that are present on the Message object are also present in the .msg file." Crucially, it is not plain text. It is a binary compound file — the same structured-storage container behind older Office documents — so it needs Outlook or a reader built to the MS-OXMSG specification to open faithfully.

How the .msg format is built inside#

The internal difference is the whole story, so it is worth one paragraph of detail. Microsoft's specification states that "the .msg File Format is based on the Compound File Binary File Format," which "provides for the concept of storages and streams, which are similar to directories and files, except that the entire hierarchy of storages and streams are packaged into a single file, called a compound file." In other words, an .msg is a tiny file system inside one file — the OLE structured-storage model Microsoft calls the Compound File Binary Format.

Inside that compound file, each Outlook property lives in its own named stream, and recipients and attachments each get their own storage. The specification names them directly: recipient storages such as "__recip_version1.0_#00000000", attachment storages such as "__attach_version1.0_#00000000", a property stream and a named-property mapping storage. This is how MSG preserves things a plain RFC 5322 message has no field for — but it is also why you cannot just read one in Notepad.

EML vs MSG at a glance#

The table below sets the two side by side on the dimensions that actually decide an export. Read it as a map of trade-offs, not a scoreboard: neither format is "losing" — each is built for a job the other handles worse.

How to choose: start with the destination#

The cleanest way through this decision is to ignore the formats for a second and name where the file is going. A message you are storing for years, or handing to someone on a Mac, has different needs from one you are dragging out of Outlook and back into Outlook next week. The fork below is the whole decision in one picture: portability and long-term readability pull toward EML, while keeping Outlook's own bookkeeping pulls toward MSG.

Once you know the destination, the format almost always chooses itself. The sections that follow walk each side in turn, then give you a lookup table of common goals so you can match your case in a few seconds.

Where EML wins#

EML's strength is that it is the lowest common denominator of email, in the best sense. Because it is the RFC 5322 format that mail servers exchange, an .eml file is the message in its true, original form — and almost every program that touches email can read it. That universality is exactly what you want for an archive that has to outlive whatever client you use today.

It also preserves the original headers verbatim. The Received chain, the DKIM-Signature, the Message-ID, the authentication results — all of it sits in the file as plain text, which matters when the point of keeping a message is to show what actually arrived and from where. Nothing has been re-interpreted into a vendor's property model; the wire form is the file.

And because MIME packs HTML, inline images and attachments into that one text file, a proper .eml is self-contained and portable. You can move it between Windows, macOS and Linux, hand it to a colleague on a different client, or re-import it into a fresh mailbox years later, and it opens. There is no vendor to depend on and no licence to worry about — the standard is public and stable.

• Opens in nearly any client or a plain text editor — no Outlook required.
• Keeps original headers as-is, which is what makes a saved message verifiable.
• Truly cross-platform: the same file works on Windows, macOS and Linux.
• An open IETF standard, so it is a safe bet for long-term archives.

Where MSG wins#

MSG wins wherever Outlook's own bookkeeping is part of what you are saving. RFC 5322 has no field for an Outlook colour category, a follow-up flag, a message class, a voting-button response, a sensitivity setting or a custom named property — but MS-OXMSG stores all of them, because it is designed to hold the full Outlook Message object, not just the parts that travelled over the wire. If those details are the reason you are exporting, MSG is the only format that keeps them.

That makes MSG the natural choice for round-tripping inside an Outlook-centric environment. Drag a message from Outlook to your desktop and it becomes an .msg; drag it back into a folder and Outlook rebuilds it with its categories and flags intact. In a records or matter-management system that expects Outlook items, MSG hands the receiving tool everything Outlook knew about the message, which a stripped-down text export cannot.

The trade-off is real and worth stating plainly: an .msg is a binary compound file, so it generally needs Outlook — or a third-party reader written to the MS-OXMSG spec — to render correctly. Send one to someone on a Mac with no Outlook and you have handed them a file they may not be able to open. MSG buys fidelity to Outlook at the cost of the universal readability EML gives you for free.

• Preserves Outlook-only metadata: categories, flags, message class, custom fields.
• Round-trips inside Outlook with nothing dropped.
• Fits Outlook-based records and matter systems that expect Outlook items.
• Trade-off: needs Outlook or an MS-OXMSG-aware reader to open faithfully.

Openness and lock-in: what each format really costs you#

Neither format costs money — the interesting difference is openness and dependency. EML is defined entirely by public IETF standards: RFC 5322 for the message and the MIME RFCs for everything richer. Those standards are implemented in essentially every mail system on earth, so an EML archive has effectively no lock-in. Any tool, on any platform, now or in a decade, can read it.

MSG is Microsoft's format, but it is not a secret one. Microsoft publishes the full MS-OXMSG specification under its Open Specifications program, which is why third-party libraries and viewers can implement it. In practice, though, faithful rendering of every Outlook property still leans on Outlook, so MSG carries a practical dependency EML does not. You are not locked out of the format — the spec is right there — but you are, for full fidelity, leaning on Microsoft's client.

One caution specific to this comparison: verify format details against the primary specifications rather than a summary, because they are living documents. The MS-OXMSG specification is versioned and revised — the published revision was 18.0, dated May 2025, when this guide was written — so check Microsoft Learn for the current one. RFC 5322 (which obsoleted the older RFC 2822) is the stable reference for EML. Both are linked in the sources below.

Who each format is genuinely for#

Pull it together with the destination-first lookup below. Find the goal that matches yours and the format falls out of it. The only row that deserves care is legal disclosure, and it gets its own note underneath, because the right answer there is set by someone other than you.

Where a mail client like AI Emaily fits — and where it doesn't#

It is worth being clear about where a tool like ours sits in this picture, because the honest answer is that it sits to one side of it. AI Emaily is an AI email client, not a file-format converter or an e-discovery platform. It does not turn your mailbox into a folder of .eml or .msg files, and if that one-off export is your task, the right tool is your desktop mail client's own "Save As" or a dedicated converter — not us. We would rather point you there than pretend otherwise.

What it does do is work with the live message instead of the exported file. AI Emaily connects Gmail, Outlook and any IMAP mailbox — all speaking the same RFC 5322 mail underneath the covers this guide just took apart — and helps you triage your inbox, draft replies in your own voice (from a Context brain and per-client profiles you set, not by scraping your old mail), and search across every account. The point is that a message you keep managed in the inbox is one you reach for a file export for less often.

We build AI Emaily, so treat that as our bias stated up front rather than hidden. Where you genuinely need portable files — a long-term archive, a mailbox migration, a formal production — reach for the standards-based export or the dedicated conversion tool, and use EML or MSG by the destination rules above. Where you want the day-to-day inbox itself handled, that is our lane, and you can see the plans and start a 7-day free trial on the pricing page.

The bottom line on EML vs MSG#

EML and MSG are not two grades of the same thing where newer wins. EML is the open, portable, standards-based form of a message — the safe default for archiving, for moving mail between programs, and for anyone who is not living inside Outlook. MSG is Outlook's own richer form, the one that keeps the categories, flags and custom fields RFC 5322 was never built to carry, at the price of needing Outlook to read it well.

Choose by where the file is going. Cross-platform, long-term, or headed to a non-Outlook recipient: EML. Outlook-to-Outlook, or you need the Outlook metadata preserved: MSG. And when the destination is a court, let the production specification decide, not your habit. Get that one question right and the format stops being a coin-flip.