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

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 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 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 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.