PST vs MBOX vs EML: the only guide a normal person needs
You exported your email from somewhere, and now you're staring at a
.pst, a .mbox, or a folder full of .eml files — and no
idea what any of it means. Here's what each one actually is, who makes it, and what you
should do with it.
PST: Outlook's single-file container
A PST ("Personal Storage Table") is Microsoft's format — the file Outlook uses to store an entire mailbox locally: mail, folders, contacts, calendar, all in one container. If you ever used desktop Outlook, you probably have one or several of these sitting on an old drive.
The gotcha: there are two incompatible generations. Outlook 97–2002 wrote ANSI PSTs, capped around 2 GB — and when an old ANSI file hits that ceiling, it can silently stop saving new mail rather than warning you. Outlook 2003 onward writes Unicode PSTs with no such cap. Worse, the new Outlook for Windows that Microsoft now installs by default can't open PST files at all — you need classic desktop Outlook, or a separate tool. We've written a full guide to opening PSTs without Outlook if that's the problem in front of you right now.
MBOX: the old plain-text one-big-file format
MBOX is one of the oldest email formats in existence — a plain text file with every message concatenated one after another, attachments encoded inline. It's not tied to any one company; it's what Google Takeout hands you when you export Gmail, and plenty of other services use it too. Its age is genuinely a feature: MBOX has been readable by software for decades and will still be readable in decades more.
The downside is the same as PST's: it's one giant opaque lump. You can't browse it in a file explorer, search it directly, or grab message #40,000 without a tool that understands the format. If you just downloaded a Takeout zip and are staring at an MBOX file for the first time, this guide walks through exactly what to do with it.
EML: one file per message
EML is different in kind, not just origin. Instead of one big container, every single email is its own ordinary file on disk. That makes it the friendliest format for archiving by a wide margin: you can browse messages in a file explorer, search them with ordinary desktop search, back them up incrementally (copy only what changed, not the whole mailbox again), and never risk one corrupted container taking down your entire mail history at once. EML is also what most serious extraction and migration tools export to — it's the common landing format, not a proprietary one.
Which one should you actually keep?
Depends on the provider, but the pattern is consistent:
- Outlook / Exchange gives you PST.
- Gmail (via Google Takeout) gives you MBOX.
- Most other webmail exports, and most migration tools, give you EML — or should.
For a long-term archive, individual EML files are the right thing to keep. A "one giant container" format like PST or MBOX is fine for transport — moving a mailbox from one place to another in a single download — but it's a poor choice for preservation, because the whole thing lives or dies as one unit, and you're permanently dependent on software that understands that specific container.
Converting between them safely
Whatever format you're starting from, the conversion should happen entirely on your own computer — never through a website that asks you to upload the file first.
Never upload your mailbox to a "free converter" site.
A PST or MBOX file is your entire email history: password resets, financial statements,
medical correspondence, years of private conversations. Any site that asks you to upload it
in exchange for a converted download should be treated as a data breach you'd be causing
yourself. Tools like Thunderbird with the ImportExportTools NG add-on, or the open-source
libpst/readpst toolchain for PSTs, run locally and don't have this
problem.
The real fix: normalize, de-duplicate, organize
Converting one file is a start, but most people don't have just one export — they have a PST from the Outlook years, an MBOX from a Takeout download, and maybe a folder of EML files from somewhere else, all overlapping. The move that actually produces a lasting archive is to normalize everything down to individual EML files, de-duplicate at the message level, and organize by person or account rather than by which export it happened to come from. When I did this across my own family's mail — 736,000 messages from a dozen sources — more than half turned out to be duplicates. If your mail is spread across several accounts rather than several file formats, this companion guide covers combining multiple accounts into one archive.
Frequently asked
What is the difference between PST and MBOX? Both are single-file containers holding an entire mailbox, but PST is Microsoft's proprietary Outlook format while MBOX is an older, open, plain-text format used by Gmail Takeout and many other services. Neither is easy to browse without dedicated software.
Is EML better than PST? For archiving, yes. One file per message means no proprietary container to depend on, ordinary file-level search and backup, and none of PST's generational quirks like the old 2 GB ANSI cap.
How do I open an MBOX file? Thunderbird with the free ImportExportTools NG add-on is the standard route — install both, import the file, and browse it like a mailbox. Never use a website that asks you to upload the file first.
Which email format is best for archiving? Individual EML files. A one-message-per-file archive survives software changes, is fully searchable with ordinary tools, and isn't at risk of a single damaged container taking your whole mail history with it.
EmailAttic reads all three — and merges them into one archive
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