EML vs MSG — Same Email, Different Packaging
Both formats hold a single email message. EML stores it as open-standard plain text; MSG wraps it in Outlook's proprietary binary container. The email inside is the same — the difference is who can open it, what metadata survives, and how easily you can inspect the contents. Need to open one right now? Drop it into the free viewer — it handles both formats, entirely in your browser.
The viewer auto-detects the format — drop either type. Nothing uploads, no sign-up.
The short version
EMLis the internet's native email format. When Gmail says "Download message", it gives you an .eml. When a mail server passes a message to the next server, it sends an EML stream. When Thunderbird, Apple Mail, or Outlook on the web saves an email to disk, the result is an .eml file. It is plain text from top to bottom — open it in Notepad and you can read the headers, the body, and even the encoded attachments.
MSGis Microsoft Outlook's desktop format. When you drag a message out of Outlook for Windows, or choose File → Save As, you get an .msg file. Under the hood it is a compound binary file — the same container format (.doc, .xls, .ppt) that Microsoft used across Office before the XML-based .docx era. It holds the same email data as EML, plus Outlook-specific properties like categories, flags, and voting buttons that have no equivalent in the internet standard.
The practical consequence: everyone can open EML; only Outlook (and dedicated parsers like our viewer) can open MSG.
Side-by-side comparison
| .eml | .msg | |
|---|---|---|
| Full name | Electronic Mail file (RFC 5322 + MIME) | Outlook Message file (MS-CFB / OLE compound document) |
| Format type | Plain text — human-readable in any text editor | Binary container — a mini filesystem with sectors and directory entries |
| Standard | Open (IETF RFC 822 → 2822 → 5322, MIME RFC 2045–2049) | Proprietary (Microsoft MS-OXMSG, published but not an internet standard) |
| Created by | Gmail, Thunderbird, Apple Mail, Outlook on the web, mail servers | Outlook for Windows (drag to desktop or File → Save As) |
| Opened by | Any mail client, any text editor, browser-based viewers | Outlook, or a parser that implements the OLE/CFB specification |
| Holds | One email: headers, text body, HTML body, attachments (MIME parts) | One email: same data plus Outlook-only properties (categories, flags, voting buttons, follow-up dates) |
| Attachment encoding | Base64 inside the text stream (+33% size overhead) | Raw binary stored in sub-streams inside the container (no encoding overhead) |
| Typical size | 5–15% smaller than MSG for the same message | 5–15% larger due to container overhead and sector padding |
| Security inspection | Easy — headers and body are plain text, grep-able, diffable | Hard — binary format requires a parser; cannot be searched with standard text tools |
| Forensic / legal use | Preferred in e-discovery and cross-platform evidence — universally parseable, header chain intact | Accepted but requires Outlook or a validated parser; header chain may be reformatted by Outlook |
| Portability | Works everywhere email works — the internet's native format | Locked to the Microsoft ecosystem; non-Outlook tools need a third-party library |
How they look on the inside
The easiest way to feel the difference: open both in a hex editor. An EML file starts with something like Return-Path: <sender@example.com> — human-readable from byte one. An MSG file starts with D0 CF 11 E0 A1 B1 1A E1 — the OLE magic bytes that identify a compound binary file.
Inside the EML, the message is laid out linearly: headers at the top, a blank line, then the MIME body parts separated by boundary markers. A single pass through the file, splitting on boundaries and decoding base64 or quoted-printable where needed, gives you the full message. The format is simple enough that people regularly write EML parsers as weekend projects.
Inside the MSG, the same data is spread across a tree of storages and streams within the OLE container. The subject is in one stream, the body in another, each attachment in its own sub-storage. Reading them requires navigating the container's FAT (file allocation table) and property sets — substantially more code than parsing EML, and impossible without a library or a deep familiarity with the [MS-CFB] and [MS-OXMSG] specifications.
When to use EML
- Sharing with non-Outlook users. EML opens in Thunderbird, Apple Mail, Gmail, any browser viewer — no questions asked. MSG will get you a reply asking what the file is.
- Archiving for long-term storage. EML is plain text built on a 40-year-old open standard. You can read it with tools that do not exist yet. MSG depends on one vendor's format specification.
- E-discovery and legal review. Most review platforms expect EML or MBOX. EML preserves the original header chain exactly as transmitted — critical for authentication analysis.
- Batch processing and scripting. EML files are text. You can grep them, diff them, split them with standard Unix tools. Try that with binary OLE containers.
- Cross-platform workflows. If the recipient might be on a Mac, a Chromebook, or a phone, EML is the safe choice.
When to keep MSG
- Preserving Outlook metadata. Categories, flags, follow-up dates, voting buttons, custom forms — none of these survive a conversion to EML. If you need them, keep the MSG.
- Staying inside the Outlook ecosystem. If everyone involved uses Outlook for Windows, MSG is the native format and double-clicking it does exactly what you expect.
- Outlook add-in integrations. Some compliance and CRM tools hook into Outlook's MSG handling. Using EML would bypass those integrations.
If you need to move a message from MSG to EML — to share with a non-Outlook user, to archive in a portable format, or to submit for legal review — our MSG to EML converter handles the extraction in your browser. The core data (headers, bodies, attachments) comes through intact; Outlook-only properties like categories and flags are discarded because EML has no place to put them.
How Outlook decides which format to use
This catches people off guard: Outlook does not always produce MSG files.
- Outlook for Windows— drag a message to the desktop → MSG. File → Save As → defaults to MSG but offers "Text Only" (headers + plain text, not a real EML) and "Outlook Message Format - Unicode" (MSG with Unicode support). There is no "Save as EML" button in desktop Outlook.
- Outlook on the web (OWA) — the download option saves as EML, because the server handles the message in internet-standard format and has no OLE container to wrap it in.
- Outlook for Mac — drag produces EML by default, because the Mac version was built on a different engine and does not use the OLE container internally.
- New Outlook (Windows) — the rebuilt version based on the web app also saves as EML when you download a message.
The trend is clear: outside of classic Outlook for Windows, Microsoft itself defaults to EML.
Converting between EML and MSG
MSG → EMLis the easy direction. The email's core data — headers, text body, HTML body, and attachments — maps directly to MIME. Our MSG → EML converter extracts it in your browser: drop the MSG file, download the EML. Outlook-specific properties (categories, follow-up flags, voting buttons) are dropped because the MIME standard has no fields for them.
EML → MSG is harder. You need to build an OLE compound file from scratch, create the correct property streams, and populate MAPI properties with the right IDs and types. Desktop tools like Outlook itself (import the EML, then drag it out as MSG) or specialized conversion software handle this. The result is a valid MSG file, but any Outlook-specific metadata has to be invented or left empty.
Practical advice: unless you specifically need MSG for an Outlook-only workflow, convert to EML rather than from it. EML is the format the rest of the world uses, and the conversion loses nothing that matters outside of Outlook.
FAQ
What is the difference between EML and MSG files?
EML is a plain-text file following the internet's standard email format (RFC 5322 + MIME). Any text editor can open it. MSG is a binary file in Microsoft's OLE/CFB compound-document format — essentially a mini filesystem inside a file. It requires a parser that understands that container. The email data inside both is the same; the packaging is not.
Which format is better, EML or MSG?
Neither is universally better. EML is more portable — it opens in any mail client, any text editor and any standards-compliant viewer. MSG preserves Outlook-specific metadata (categories, flags, follow-up dates, voting buttons) that EML cannot carry. If you need maximum compatibility, use EML. If you need every Outlook property intact, use MSG.
Can I convert EML to MSG or MSG to EML?
Yes. MSG to EML is straightforward: the core email data (headers, body, attachments) maps cleanly to MIME. Our MSG to EML converter does this in your browser. EML to MSG is harder because the OLE container has to be built from scratch and some Outlook-only properties have no source data — the result is a valid MSG but may lack calendar flags and categories the original never had.
Why does Outlook save as MSG instead of EML?
Outlook for Windows uses MSG as its native drag-and-drop format because MSG can carry every property Outlook tracks internally — not just the email itself but also read/unread state, categories, custom forms, and links to other Outlook items. EML cannot represent any of those. Outlook on the web, by contrast, downloads messages as .eml because it does not have those desktop-only properties to preserve.
Can Gmail open MSG files?
No. Gmail and most webmail services only understand EML (the internet standard their own servers already use). If someone sends you an .msg attachment, you need either Outlook or a third-party viewer. Our viewer opens both .eml and .msg files in the browser — drop the file and it auto-detects the format.
Are EML files safe to open?
An EML file is inert text and cannot execute by itself. The risk comes from what renders it: the HTML body can contain tracking images, and attachments can carry anything an email carries. The same applies to MSG files, with the added consideration that MSG files are binary and harder to inspect manually. Our viewer sandboxes the HTML, blocks remote resources by default, and runs entirely in your browser.
Which file is smaller, EML or MSG?
For the same email, EML is typically 5–15% smaller. The base64 encoding in EML inflates binary attachments by about 33%, but the MSG container adds its own overhead — directory entries, property streams, padding to 64-byte sector boundaries. For a plain-text message with no attachments, the difference is negligible. For a message with a 5 MB PDF, EML is usually a few hundred KB lighter.