The short version
- MBOX — plain-text, many emails per file, Unix-origin, universal support.
- PST — binary, email + calendar + contacts + tasks in one database, Microsoft Outlook-specific.
- EML — plain-text, one email per file, mostly used for saving individual messages.
If you're picking a format for long-term archival and cross-platform compatibility, pick MBOX. If you're stuck with an Outlook export, you have PST. If you're saving individual emails for later reference, EML.
MBOX — the Unix-origin plain-text container
Structure
Many email messages concatenated into a single plain-text file. Each message starts with a line beginning From (note the trailing space, no colon), followed by RFC 5322 headers, an empty line, and the message body.
Origin and history
Introduced in 1975 by the Unix mail utility. Originally just a user's on-disk inbox at /var/mail/username. Adopted by essentially every Unix-based mail system that followed.
Who uses it
- Gmail (Google Takeout exports as MBOX)
- Apple Mail (Mailbox › Export Mailbox produces MBOX bundles)
- Mozilla Thunderbird (every folder is backed by an extensionless MBOX file)
- ProtonMail, Fastmail, and most privacy-focused mail providers
- Postfix, Dovecot, qmail, sendmail, and most Unix mail servers
Strengths
- Human-readable (open in any text editor)
- Standards-based and well-documented (RFC 4155)
- Universal tool support
- No vendor lock-in
- Works with grep, awk, and other Unix text tools
Weaknesses
- Stores only email — not calendar, contacts, or tasks
- Single-writer file format (concurrent writes can corrupt)
- Attachments encoded inline, bloating file size
- Some metadata (read/unread status, flags, labels) doesn't round-trip cleanly
For a deeper dive see the MBOX file primer.
PST — Microsoft Outlook's binary database
Structure
A proprietary binary file format that stores email, calendar events, contacts, tasks, and notes in a single structured database. Designed for Outlook's combined personal-information-manager model.
Origin and history
Introduced by Microsoft in 1997 with Outlook 97. Has gone through multiple format generations (ANSI PST for older versions, Unicode PST since Outlook 2003). Microsoft publishes the format specification (MS-PST) but third-party tooling is still more limited than for plain-text formats.
Who uses it
- Microsoft Outlook users (desktop versions, all platforms)
- Exchange server exports for individual mailboxes
- Enterprise email archives in Microsoft environments
- eDiscovery productions from Microsoft 365 tenants
Strengths
- Holds complete PIM data (email + calendar + contacts + tasks)
- Efficient storage (compression and indexing)
- Native to the largest enterprise email platform
- Handles very large mailboxes (50+ GB in modern Unicode PSTs)
Weaknesses
- Binary format — not human-readable without Outlook or a dedicated viewer
- Microsoft-specific — Mac/Linux tools are limited
- Corruption in the database can be painful to recover from
- Proprietary specification slows third-party innovation
EML — the single-message plain-text format
Structure
Plain text, one email per file. The file is essentially a single RFC 5322-formatted message — the same format the email was transmitted in. MIME-encoded attachments appear inline within the body.
Origin and history
Evolved informally from early email standards and became commonly used when Microsoft Outlook Express (and later Windows Mail) adopted it as the save-single-message format. The .eml extension is recognized by most mail clients as "a single saved email".
Who uses it
- Outlook Express (historical), Windows Mail, Apple Mail (when saving individual messages)
- Many webmail clients when you "Download original" on a single message
- Forensic tools that extract messages one at a time
- Email services that want to give you a durable copy of one specific email
Strengths
- Simple, plain-text, standards-compliant
- Each message is a self-contained file
- Opens in most mail clients with a double-click
- Easy to share a single message as a file
Weaknesses
- One file per message — archives of thousands of emails become thousands of files
- No built-in folder/mailbox organization
- Same attachment-bloat issue as MBOX (inline MIME encoding)
Side-by-side comparison
Adjacent formats worth knowing
OST (Outlook Offline Storage)
Sibling to PST. Same binary database structure, but OST is Outlook's local cache of an Exchange/IMAP account — not a permanent archive. Typically tied to a specific Outlook profile and can be re-downloaded from the server if lost.
MSG (Outlook single message)
Microsoft's single-message format, equivalent to EML for Outlook. Binary. Stores one email with Outlook-specific metadata.
MBX (Eudora)
Eudora's proprietary mailbox format. Similar concept to MBOX but different structure. Eudora is discontinued; MBX archives still exist for users migrating off.
Maildir
A format used by modern Unix mail servers (Dovecot, qmail) that stores one email per file in a directory structure. Fixes MBOX's concurrent-write problem. Rarely seen in consumer archives but common in server-side storage.
How to convert between them
Any format → PDF
For PDF output on Mac, the cleanest paths:
- MBOX → PDF: use MBOX to PDF directly.
- EML → PDF: depending on volume, either Apple Mail import + print-to-PDF (small batches) or a dedicated EML-to-PDF converter.
- PST → PDF: convert PST → MBOX first with Aid4Mail or a similar tool, then MBOX → PDF.
Between MBOX, PST, and EML
Most dedicated email converters (Aid4Mail, SysTools, BitRecover) round-trip between these three plus others. If you already have the source as MBOX, keep it as MBOX for Mac workflows. If you have PST and need something Mac-friendly, convert to MBOX.
Picking a format for new archives
- For long-term permanence, pick MBOX. Human-readable, universal, standards-based, will still open in 20 years.
- For sharing a single email, pick EML. Opens in any mail client.
- Avoid PST for new archives unless you're already in an Outlook environment. It's a fine active format; it's a lock-in format for archives.
- For ultimate permanence, convert to PDF. Format-independent, opens without any mail tooling, stable for decades.
Frequently asked questions
What is the difference between MBOX, PST, and EML?
MBOX: plain text, many emails per file, Unix origin. PST: binary, email + calendar + contacts, Outlook. EML: plain text, one email per file. All widely supported, different typical uses.
Which format is most portable?
MBOX and EML. Both plain-text, standards-based, supported everywhere. PST is Outlook-specific.
Can I convert between MBOX, PST, and EML?
Yes. Most dedicated email converters round-trip between them. For Mac workflows ending in PDF, use MBOX to PDF for MBOX directly.
Why does Outlook use PST instead of MBOX?
PST predates widespread cross-platform portability and was designed around Outlook's combined email + calendar + contacts + tasks model. MBOX only holds email.
Which format is best for legal discovery?
PDF for production. Source MBOX/PST/EML for authentication.
What is MBX?
Eudora's proprietary mailbox format. Similar to MBOX but different structure. Eudora is discontinued.
Related reading
- What is an MBOX file? — detailed primer
- How to convert MBOX to PDF on Mac
- Best MBOX converter for Mac (2026)