EML vs MSG: the differences, and which to save as
EML vs MSG in one table. .msg keeps Outlook-only data such as Exchange senders, flags and appointments; .eml opens in any mail app. Which to choose when saving from Outlook.
Both formats store a single email message in a single file. The difference is that .eml is a published internet standard and .msg is a Microsoft implementation detail that escaped into the wild.
Short answer: which should you save as?
- Save as .eml if the message has to open somewhere other than Outlook: a Mac, a phone, Gmail, Thunderbird, or an archive system.
- Keep .msg if the file is evidence or a record of an Outlook item. It keeps Exchange sender details, flags and categories, and it is the only one of the two that can hold an appointment, contact or task.
- Not sure? Keep the .msg. You can convert it to .eml at any time; converting back cannot restore what .eml dropped.
Which format Outlook gives you
Often you do not get to choose. Classic Outlook for Windows saves .msg, whether you use File, Save As or drag a message to the desktop, and has no .eml option. Outlook for Mac produces .eml when you drag a message out, and the new Outlook for Windows and Outlook on the web download messages as .eml.
At a glance
| .msg | .eml | |
|---|---|---|
| Owner | Microsoft (Outlook) | IETF standard (RFC 5322 / MIME) |
| Structure | Binary OLE2 compound file | Plain text with MIME parts |
| Readable in a text editor | No | Yes |
| Opens in Apple Mail / Thunderbird | No | Yes |
| Stores non-mail items | Yes — contacts, appointments, tasks | No — mail only |
| Exchange-specific data | Preserved | Lost |
| Typical size | Larger | Smaller |
What .msg preserves that .eml cannot
.msg is a snapshot of an Outlook item, not of an internet message, so it keeps things that never travel over SMTP:
- Message class — whether the item is a note, appointment, contact or task.
- Exchange addresses — the internal
/O=…/OU=…/CN=…distinguished names of internal senders and recipients, which is often the only record of who an internal address belonged to. - Voting responses, flags, categories and follow-up state.
- Drafts — an unsent message has no internet headers at all, so converting it loses the routing metadata that was never there.
What .eml gives you that .msg does not
- It opens everywhere. Mail clients on every platform read it natively; see how to open an .eml file.
- It is greppable. Being plain text, it can be searched, diffed, and processed with ordinary command-line tools.
- It is smaller, because it does not carry the compound-file overhead or duplicate RTF and HTML copies of the same body.
- It is future-proof. A documented, text-based standard will still be readable when today's applications are gone.
Why the same message is a different size in each format
Convert a message and the file usually shrinks, sometimes by half. That is not data loss in the way it first appears — it is mostly the duplication going away.
A .msg commonly stores the body two or three times over: as HTML, as compressed RTF, and as plain text. It also carries MAPI bookkeeping that internet mail has no field for. An .eml keeps one MIME tree, so the alternatives collapse to what a mail client actually needs.
What an archive or e-discovery system expects
If the reason for converting is that something downstream has to ingest the message, the two formats are not equally welcome:
- Archives and case management systems almost universally accept
.eml, because it is the format mail travels in. Support for.msgis common but not guaranteed, and is often an extra-cost module. - Full-text indexers read
.emldirectly. Indexing a.msgrequires a parser that knows about compressed RTF, and an indexer without one records an empty body while reporting success. - Long-term preservation favours
.emlfor a plain reason: it is text you can read with any editor in twenty years. A.msgneeds a working OLE2 implementation to make any sense at all.
When to convert .msg to .eml
Convert when you need to:
- open a message on a Mac, Linux machine or phone without Outlook;
- import messages into a non-Microsoft mail client or archive;
- store correspondence long-term in a format that will remain readable;
- hand a message to someone who does not use Outlook.
Keep the original .msg as well when the item is evidence, when it is an appointment or contact rather than mail, or when Exchange-internal sender details matter.
How to convert
Open the file in the viewer and choose Export → Save as .eml. The conversion runs in your browser and produces a MIME message with the body (as both plain text and HTML) and every attachment re-encoded as base64 parts, with inline images keeping their Content-ID so they still display in the body.
Common questions
Should I save Outlook emails as .eml or .msg?
Save as .eml if the message has to open outside Outlook: on a Mac, a phone, Gmail, Thunderbird or an archive system. Keep .msg if it is evidence or an appointment, contact or task, because .msg keeps Outlook-only data that .eml drops. If unsure, keep the .msg: you can convert it to .eml later, but not the other way round without loss.
Is .eml or .msg better for archiving?
For long-term archiving, .eml. It is a documented plain-text standard that any mail client, archive or search tool can read. Keep the .msg as well when the message may be needed as evidence, since it holds Exchange sender details, flags and categories that .eml has no place for.
Can Outlook open .eml files?
Yes. Outlook on Windows and Mac opens .eml files: double-click the file or drag it into Outlook. Most other mail apps open .eml too, which is the main reason to prefer it for sharing.
Does converting .msg to .eml lose attachments or images?
No. Attachments are carried over as MIME parts, and inline images keep their Content-ID so they still appear in the body. What is lost is Outlook-only data: internal Exchange addresses, flags, categories and voting state.
Why is the .eml smaller than the .msg?
A .msg often stores the body two or three times, as HTML, compressed RTF and plain text, plus Outlook bookkeeping. The .eml keeps one copy of each part. The message content is not lost; the duplication is.
How do I convert .msg to .eml without Outlook?
Open the .msg in the MsgLens viewer at msglens.app and choose Export, then Save as .eml. The conversion runs in your browser; the file is not uploaded.