---
title: "MBOX, EML, PST, OLM: Email Archive Formats Explained"
description: "What's actually inside an email archive? A plain-language tour of the four formats you'll meet — MBOX, EML, PST and OLM — and how to read each one on a Mac."
url: https://mboxviewerpro.com/blog/email-archive-formats-explained/
language: en
updated: 2026-06-11
source: Mbox Viewer
---

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`mbox` `eml` `formats`

# MBOX, EML, PST, OLM: Email Archive Formats Explained

What's actually inside an email archive? A plain-language tour of the four formats you'll meet — MBOX, EML, PST and OLM — and how to read each one on a Mac.

[David Carrero](https://mboxviewerpro.com/author/) · June 11, 2026

Sooner or later everyone ends up with a mysterious email archive: a Google Takeout download, an old work backup, a file IT handed over when you left a job. The extension tells you a lot about what you can do with it. Here’s the field guide.

## MBOX — the open standard

[**MBOX**](https://mboxviewerpro.com/glossary/mbox/) is the oldest and most widely supported archive format. It’s beautifully simple: every message in a mailbox, concatenated into one plain-text file, each message starting with a line that begins `From` . Because it’s open and text-based, it has become the *lingua franca* of email exports:

-   **Google Takeout** delivers Gmail as MBOX
-   **Apple Mail** and **Thunderbird** export to it
-   Classic clients — Eudora, Entourage, Netscape, Evolution, Claws Mail and many more — store or export mail this way

**How to read it on a Mac:** open it with Mbox Viewer — any size, read-only, fully offline. Note that some sources need one small step first: Apple Mail exports a `.mbox` *folder* whose inner file named `mbox` is the actual archive, and Thunderbird’s own mailbox files carry no extension, so you copy them and add `.mbox`.

## EML — one file per message

[**EML**](https://mboxviewerpro.com/glossary/eml/) is a single email as a file: headers, body and attachments, in the raw MIME format defined by the internet’s email standards. Outlook, Thunderbird and most clients can save individual messages as `.eml`, and many systems (ticketing tools, scanners, servers) produce them in bulk.

**How to read it on a Mac:** Mbox Viewer opens `.eml` files directly, alongside MBOX. A folder full of EMLs is just as browsable as a single archive.

## PST — Outlook for Windows

[**PST**](https://mboxviewerpro.com/glossary/pst/) is Microsoft Outlook’s personal storage format on Windows: a proprietary database holding mail, calendar, contacts and more. It’s compact and fast inside Outlook — and opaque outside of it.

**How to read it on a Mac:** not directly with an mbox reader, because PST isn’t mbox. Convert it to MBOX first with a converter tool, then open the result with Mbox Viewer.

## OLM — Outlook for Mac

**OLM** is the export format of Outlook for Mac — same idea as PST, different container, equally proprietary.

**How to read it:** same recipe — convert to MBOX with a converter, then open the converted file.

## The cheat sheet

| Format | What it is | Open on a Mac |
| --- | --- | --- |
| MBOX | Whole mailbox, open standard | Mbox Viewer, directly |
| EML | One message per file, MIME | Mbox Viewer, directly |
| PST | Outlook (Windows) database | Convert to MBOX first |
| OLM | Outlook for Mac export | Convert to MBOX first |

If you get to choose your export format, choose MBOX: it’s open, future-proof, and twenty years from now it will still be plain text you can read with whatever tools exist then.

## Open your archive with Mbox Viewer

Native Mac and Windows app. Streams MBOX and EML files of any size, fully offline.

[Mac App Store](https://apps.apple.com/app/mbox-viewer-pro/id6759237715) [Microsoft Store](https://apps.microsoft.com/store/detail/9NW3GVFG7DDB)