--- title: "A Field Guide to Email Storage Formats: History, Anatomy and a Comparison" description: "How email is actually stored on disk: MBOX, Maildir, EML, PST, OST, OLM, MSG and Eudora. Their history, how they are built and a side-by-side comparison." url: https://mboxviewerpro.com/blog/email-formats-history-and-anatomy/ language: en updated: 2026-06-18 source: Mbox Viewer --- [All posts](https://mboxviewerpro.com/blog/) `formats` `mbox` `pst` `history` # A Field Guide to Email Storage Formats: History, Anatomy and a Comparison How email is actually stored on disk — MBOX, Maildir, EML, PST, OST, OLM, MSG, NSF and historical formats like Eudora. Their history, how they're built, what each one is good for, and a side-by-side comparison. [David Carrero](https://mboxviewerpro.com/author/) · June 18, 2026 Every email program has to answer the same question: *where do the messages actually go on disk?* Half a century of answers has produced a small zoo of formats — some open and beautifully simple, others proprietary databases you can’t read without the app that made them. This is a tour of the ones you’ll meet, how they’re built, where they came from, and how they stack up. Broadly, they fall into three families: 1. **Open, text-based containers** — one file holds many messages (MBOX), or one file per message in a folder (Maildir). Human-readable, vendor-neutral. 2. **One message per file** — a single message as a standalone file (EML, MSG). 3. **Proprietary databases** — a binary store holding mail plus calendar, contacts and state (PST, OST, OLM, NSF). Compact inside their app, opaque outside it. ## The open, text-based formats ### MBOX — the lingua franca [**MBOX**](https://mboxviewerpro.com/glossary/mbox/) dates back to the early Unix mail systems of the 1970s. The idea is disarmingly simple: concatenate *every* message in a mailbox into one plain-text file, and mark where each one starts with a line beginning `From` (the “From\_ line”, with a space, not the `From:` header). Headers, body and [attachments](https://mboxviewerpro.com/glossary/attachment/) — encoded as text — all live inline. That simplicity hides a famous wrinkle: what happens when a message body itself contains a line starting with “From ”? Different answers gave rise to **variants** — `mboxo`, `mboxrd`, `mboxcl` and `mboxcl2` — which escape (or don’t) that sequence differently. In practice modern tools read them all. MBOX is what **Google Takeout**, **Apple Mail**, **Thunderbird** and most classic clients export, which makes it the closest thing email has to a universal archive format. ### Maildir — one file per message Created for the **qmail** server in 1995, [**Maildir**](https://mboxviewerpro.com/glossary/maildir/) takes the opposite approach: every message is its own file inside a folder, split across `tmp/`, `new/` and `cur/` subdirectories. Its great virtue is **safety without locking** — two processes can deliver mail at the same time without corrupting a shared file, the classic risk with MBOX. It’s the native format of servers like Dovecot and Courier. The trade-off is millions of tiny files, which some filesystems dislike. ### EML — a single message, the way the internet defines it [**EML**](https://mboxviewerpro.com/glossary/eml/) is one message saved exactly as it travels: the raw [MIME](https://mboxviewerpro.com/glossary/mime/) structure defined by the email RFCs (822 → 2822 → 5322). [Headers](https://mboxviewerpro.com/glossary/header/) at the top, then the body and attachments encoded in [MIME](https://mboxviewerpro.com/glossary/mime/) parts. Because it *is* the on-the-wire format, almost everything can produce and read it — Outlook, Thunderbird, ticketing systems, scanners and mail servers. A folder of `.eml` files is the simplest possible archive. ## The proprietary databases ### PST — Outlook’s personal store on Windows [**PST**](https://mboxviewerpro.com/glossary/pst/) (Personal Storage Table) is Microsoft Outlook’s on-disk database on Windows, built on the MAPI model. It holds far more than mail — calendar, contacts, tasks, notes — in one binary file. The original **ANSI** PST (Outlook 97–2002) capped out at **2 GB** and was prone to corruption near that limit; the **Unicode** PST (Outlook 2003+) raised it to **20–50 GB**. Fast and compact inside Outlook, but useless to other apps without conversion. ### OST — the offline cache **OST** (Offline Storage Table) is PST’s sibling: a *cached* copy of a mailbox that lives on an Exchange or Microsoft 365 server. It exists so Outlook works offline and re-syncs later. Crucially, an OST is tied to its account and profile — it isn’t a portable archive, and orphaned OST files can be hard to open at all. ### OLM — Outlook for Mac **OLM** is the export/archive format of Outlook for Mac. Same intent as PST, different container — a proprietary bundle that, like PST, needs converting before anything but Outlook can read it. ### MSG — a single Outlook message [**MSG**](https://mboxviewerpro.com/glossary/msg/) is one message exported from Outlook, stored as an OLE “compound file” (a mini-filesystem inside a file) carrying MAPI properties. It’s the Windows counterpart to EML, but binary and Microsoft-specific. ### NSF — Lotus Notes / HCL Domino **NSF** (Notes Storage Facility) is the database behind IBM/Lotus Notes (now HCL Domino) — an entire application platform, not just mail. NSF archives still surface in long-running enterprises and, like the others here, require dedicated tools to extract. ## The historical ones - **Eudora** (1988–2006) was the dominant client of the early internet era. It stored mail in **`.mbx`** mailbox files — essentially MBOX — paired with a **`.toc`** table-of-contents index. Because the body is MBOX-like text, Eudora archives are usually recoverable today. - **Outlook Express** used **`.dbx`** files (one per folder) on Windows through the late 1990s and 2000s; its successor **Windows Mail / Live Mail** moved to individual **`.eml`** files. - **Netscape/Mozilla** mail, **Evolution**, **Claws Mail**, **Entourage** and others stored or exported MBOX — which is exactly why MBOX remains so widely readable. ## Side-by-side | Format | Structure | Open? | Portable archive? | Origin | | --- | --- | --- | --- | --- | | MBOX | One text file, many messages | ✅ Open | ✅ Excellent | Unix, 1970s | | Maildir | One file per message, in folders | ✅ Open | ✅ Good | qmail, 1995 | | EML | One message, raw MIME | ✅ Open | ✅ Excellent | Internet RFCs | | MSG | One message, OLE compound | ❌ Proprietary | ⚠️ Limited | Microsoft | | PST | Binary database (mail + PIM) | ❌ Proprietary | ⚠️ Convert first | Outlook (Win) | | OST | Cached server mailbox | ❌ Proprietary | ❌ Account-bound | Outlook/Exchange | | OLM | Proprietary bundle | ❌ Proprietary | ⚠️ Convert first | Outlook (Mac) | | NSF | Application database | ❌ Proprietary | ⚠️ Convert first | Lotus Notes | ## What to choose for the long term For *archiving* — keeping mail readable for decades — the open, text-based formats win every time. MBOX and EML have no vendor, no licence and no database engine to go obsolete: in twenty years they’ll still be plain text any tool can open. That’s why, if you ever get to pick an export format, **MBOX** (or a folder of **EML** files) is the safe choice, and why converting PST/OLM to MBOX future-proofs an Outlook archive. Once your mail is in MBOX or EML, [Mbox Viewer](https://mboxviewerpro.com/) opens it on Mac and Windows — any size, read-only, fully offline. For the practical “which file can I open and how” version of this guide, see [MBOX, EML, PST, OLM: Email Archive Formats Explained](https://mboxviewerpro.com/blog/email-archive-formats-explained/); to turn an Outlook file into MBOX, see [how to convert PST/OLM to MBOX](https://mboxviewerpro.com/guides/convert-outlook-pst-olm-to-mbox/). ## 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)