MBOX vs PST: what’s the difference and which do you need?

What is an MBOX file?

MBOX is one of the oldest email storage formats, dating back to early Unix mail systems. An MBOX file is plain text: every message in a mail folder is stored one after another in a single file. Each message begins with a separator line that starts with From (the word “From” followed by a space), then the usual headers and the message body.

Because it is just text, you can open an MBOX file in any text editor and read the headers, even if attachments show up as long blocks of encoded characters. That simplicity is the format’s biggest strength.

A few things worth knowing

• One file per folder. An MBOX file normally holds one mailbox, such as Inbox or Sent. A full mail account is a set of MBOX files, one per folder. Google Takeout is the exception: it puts all your Gmail into one large file and records each message’s labels in an X-Gmail-Labels header.
• Slight variants exist. There is no single official standard, so there are a few flavors (known as mboxo, mboxrd, mboxcl and mboxcl2). They differ mainly in how they deal with message lines that happen to start with “From ”. Good tools handle all of them, and you rarely need to care.
• Flags live in headers, or nowhere. Read, replied and flagged status is stored in extra headers such as Status or Thunderbird’s X-Mozilla-Status, if the app records it at all. Some exports lose this information.
• The extension varies. Files may end in .mbox, .mbx, or have no extension at all. Thunderbird’s folder files are simply called Inbox, Sent and so on. Apple Mail exports a folder named like Inbox.mbox with a file called mbox inside.

What is a PST file?

PST (Personal Storage Table), also called an Outlook Data File, is Microsoft’s format for storing Outlook items on your computer. It is a binary database rather than text: you can’t read it in a text editor. Microsoft publishes the full specification as part of its open specifications program (MS-PST), so it is proprietary but documented.

A single PST can hold much more than email: contacts, calendar items, tasks and notes, all inside a complete folder hierarchy. One file can represent an entire mailbox, which is why PST is popular for archives, backups and moving mail between computers.

ANSI vs Unicode PST

There are two generations. The older ANSI format (Outlook 97 to 2002) is limited to about 2 GB and handles non-English text poorly. The Unicode format, introduced with Outlook 2003, supports far larger files and any language. Every current version of Outlook creates Unicode PSTs by default.

PST is not the same as OST

Outlook also uses .ost files. An OST is an offline cache of a mailbox that lives on a server (Microsoft 365, Outlook.com or Exchange). It lets Outlook work offline and resyncs with the server. You can’t simply open an OST in another Outlook profile or on another computer; to take mail with you, you export it to a PST.

Which apps use which format?

In practice, the app you use decides the format you get:

Notice the gap: Outlook does not open MBOX files, and none of the common MBOX apps open PST files. That is the main reason people end up converting.

For the new Outlook for Windows, classic Outlook remains the reliable way to open or import a PST. Outlook for Mac’s import options have changed over time; our PST import guide covers the current steps for each version.

Size limits

MBOX has no size limit in the format itself. The practical limits come from elsewhere: a single 20 GB file is slow to open, search and copy, and some apps struggle with very large MBOX files. Older or removable drives formatted as FAT32 can’t store any file larger than 4 GB, which can quietly break a big export.

PST files are limited by Outlook. An ANSI PST tops out at about 2 GB. A Unicode PST has a default limit of 50 GB in Outlook 2010 and later, which administrators can raise with registry settings. Even below the limit, very large PSTs get slower to work with. For the details, see Outlook PST size limits explained.

Robustness, portability and search

Corruption and recovery

MBOX is largely append-only: new mail is added to the end of the file. If something goes wrong, the damage tends to stay local, and because the file is plain text, most of the mail can usually be recovered even from a damaged file.

A PST is a structured database, so damage in the wrong place can make Outlook refuse to open it. Microsoft includes the Inbox Repair Tool (scanpst.exe) with classic Outlook for Windows to scan and fix damaged PST files. It works well in many cases, but it is still a good idea to keep backups of important PSTs.

Portability

MBOX is the more universal format. Almost every open-source mail app, many archiving tools and scripting languages can read it, on any operating system. PST is the standard in the Microsoft world: Outlook, Microsoft 365 import tools and most corporate archiving and eDiscovery systems accept it.

Search

Neither format has a built-in search index. Thunderbird and Apple Mail build their own indexes on top of MBOX files, and Windows Search indexes PST files that are open in Outlook. In everyday use, how well you can search depends more on the app than the file format.

MBOX vs PST at a glance

Neither format is “better” in general. MBOX is simpler and more open; PST is richer and fits the Outlook and Microsoft 365 ecosystem. The right one is the one your email app and your organization use.

When to convert MBOX to PST, and when not to

Converting makes sense when

• You are moving to Outlook from Gmail, Thunderbird or Apple Mail and want your old mail inside Outlook, with folders.
• Your company requires PST for archiving, legal hold or eDiscovery imports.
• You are importing into Microsoft 365. Administrators can upload PST files with Microsoft’s PST Import Service to load old mail into users’ mailboxes on the server.

You probably don’t need to convert when

• You are staying with Thunderbird or Apple Mail; they read MBOX directly.
• You just want an archive of a Gmail account. A Takeout MBOX file is a perfectly good long-term backup, and you can open it later with free tools.

If you do need a PST, there are several routes, from free do-it-yourself options to paid converters; our comparison of four ways to convert MBOX to PST walks through them honestly. MoveMyInbox is one option: you upload your MBOX files in the browser and download a Unicode PST that keeps folders (or Gmail labels as folders), attachments and read and flagged status. It converts MBOX to PST only, not the other way around. See how it works.

Frequently asked questions