Email file formats explained: EML, MSG, MBOX, Maildir, EMLX, PST, OST and OLM

First, the big difference: how much mail is in one file

The most useful way to tell email formats apart is to ask how much mail a single file holds:

• One file per message: EML, MSG and EMLX. Each email is its own file, like a letter in an envelope.
• One file (or folder of files) per mail folder: MBOX holds a whole folder, such as Inbox, in one file. Maildir uses one directory per folder, with one file per message inside.
• One file per mailbox: PST, OST and OLM hold an entire account, with its folder tree, in one file.

The second question is whether the format is open, readable by many apps, or tied to one vendor’s apps. Open formats are easier to move and keep readable for decades.

EML and MSG: one message per file

EML

An EML file is a single email saved exactly as it travels over the internet: plain-text headers (From, To, Subject, Date) followed by the body, with attachments encoded as text inside the same file. That layout is the standard Internet Message Format (RFC 5322) with MIME for attachments, so EML isn’t owned by anyone.

Almost everything can create or open one. Gmail’s Download message option saves an EML; Outlook on the web’s Download option can save EML or MSG; Apple Mail saves EML when you choose File → Save As with the Raw Message Source format or drag a message to the desktop; Thunderbird saves messages as .eml too. Outlook, Thunderbird and Apple Mail can all open them, and even a text editor can show you the headers.

MSG

An MSG file is Outlook’s format for a single item. Microsoft documents it publicly (as MS-OXMSG, “Outlook Item (.msg) File Format”). It is a binary file rather than text, and it can hold not only an email but an appointment, contact or task, with Outlook-specific properties such as categories and flags.

Classic Outlook for Windows creates MSG files when you save a message with File → Save As or drag one out of Outlook to a folder. Outlook opens them with a double-click. Outside the Microsoft world support is patchy: Apple Mail and Thunderbird don’t open MSG files natively, so recipients without Outlook may need a viewer or converter.

MBOX and Maildir: the open formats

MBOX

MBOX stores every message in a mail folder one after another in a single plain-text file. It is the export format of Google Takeout and Apple Mail, and Thunderbird’s default way of storing each folder on disk. The format dates back to early Unix mail systems; RFC 4155 registers it as a media type but notes that it never had an authoritative specification, so a few slightly different variants exist.

Because it is plain text and widely supported, MBOX is one of the most portable formats there is. It is also the one Outlook can’t open. We compare the two in depth in MBOX vs PST, and how to open an MBOX file lists free ways to read one.

Maildir

Maildir is the other classic open format, mostly used on mail servers and Linux mail apps. Instead of one big file, each folder is a directory with three subfolders, cur, new and tmp, and every message is a separate plain-text file inside them. Read and replied status is recorded in each file’s name.

Thunderbird also offers an optional “File per message (maildir)” storage type, though Mozilla describes it as not quite full Maildir. Most people only meet Maildir when a hosting company or server admin hands them a backup: a folder full of small files with odd, long names.

EMLX: how Apple Mail stores mail on your Mac

Apple Mail keeps its own mail store in your user Library, in ~/Library/Mail, inside a versioned folder (such as V10) and a folder per account and mailbox. Each message is a separate .emlx file. An EMLX is essentially an EML with a little extra wrapped around it: a line giving the message’s length at the top and some Apple metadata, such as flags, at the end. Messages whose attachments are stored separately appear as .partial.emlx files.

Apple doesn’t publish a specification for EMLX, and it is meant for Mail’s own use rather than for sharing. Treat it as internal storage: don’t copy or edit those files by hand. To take mail out of Apple Mail, select the mailboxes and use Mailbox → Export Mailbox, which saves standard MBOX packages such as Inbox.mbox. Our Apple Mail guide walks through it.

PST, OST and OLM: Outlook’s mailbox files

PST

A PST (Outlook Data File) holds an entire mailbox in one binary file: email, contacts, calendar, tasks and notes, with the full folder tree. Microsoft documents the format (MS-PST). Classic Outlook for Windows creates, opens, imports and exports PSTs; Outlook for Mac can import them (the new Outlook for Mac since version 16.110, June 2026); and Microsoft 365 can import them on the server side. A PST is portable between Outlook installations, but few non-Microsoft apps read it.

OST

An OST (Offline Outlook Data File) looks similar but is only a local cache of a mailbox that lives on a server, such as Microsoft 365, Exchange, Outlook.com or IMAP. It works only with the account and profile that created it, so it isn’t portable at all. To take that mail elsewhere, export it to a PST. Our article on OST vs PST explains the details.

OLM

OLM is the archive format of Outlook for Mac. In legacy Outlook for Mac, Tools → Export saves email, contacts, calendar and other items to a single .olm file, and Import reads it back. It is a Mac-only format: Outlook for Windows can’t open OLM files. Microsoft currently documents OLM export only for legacy Outlook for Mac, so check which version you are running before you rely on it.

All eight formats at a glance

Which format should I use?

For archiving old mail

Choose an open format if you can. MBOX is the usual pick: easy to store and readable by free tools on any system. A Gmail Takeout export is already MBOX. If you live in Outlook and want to search the archive there, a PST is fine too; keep it below the size limit and keep a backup. Either way, keep the original export as well as any converted copy. See how to archive an email account before closing it.

For moving mail into Outlook

Use PST. It is the one format classic Outlook for Windows imports with folders intact, and legacy Outlook for Mac and Microsoft 365 accept it too. If your mail is currently in MBOX (Gmail, Thunderbird, Apple Mail), it needs converting first; our PST import guide covers the step after that.

For sharing one message

If you need a file, EML is the safest choice because nearly any mail app can open it. Use MSG only if the recipient uses Outlook on Windows or has asked for it. For someone who just needs to read it, a PDF printout is simplest, though it loses the full headers and the original attachments.

For a legal or eDiscovery handoff

Ask first. Lawyers, eDiscovery vendors and review platforms usually specify the format they want, and some care about preserving full headers and metadata exactly as they were. Microsoft’s own eDiscovery tools in Microsoft Purview export mail as PST files or as individual MSG files, and PST is widely expected in corporate settings. Whatever you hand over, don’t open and re-save messages, keep the original export untouched, and record how and when it was made.

Converting between formats

Some conversions are built into mail apps: Apple Mail exports MBOX and classic Outlook exports PST. Others need a separate tool, and quality varies, so check that folders, attachments and dates survive before you delete anything.

MoveMyInbox handles one of those conversions: MBOX to PST. You upload mbox files from Gmail Takeout, Thunderbird or Apple Mail in your browser and download a single Unicode PST that keeps folders (or Gmail labels as folders), attachments and read and flagged status. It doesn’t convert OST, EML, MSG, OLM or anything else. See how it works, or compare the different ways to convert MBOX to PST.

Frequently asked questions