.msg vs .eml — what is the difference?
A side-by-side comparison of Outlook .msg and standard .eml email files: structure, compatibility, what each preserves, and when to convert between them.
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.
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.
- 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.
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.