Introduction to Email File Formats
Every time you send or receive an email, the data has to be stored somewhere. The way that data gets saved — the file format — determines which email clients can read it, how portable it is, and what happens when you need to move your messages from one platform to another.
Understanding email file formats is not just an academic exercise. If you are migrating from Outlook to Thunderbird, archiving corporate emails for compliance, or recovering messages from an old backup, the format of your email files directly affects what tools you need and how smooth the process will be.
This guide walks through the seven most common email file formats in use today. We will cover what each format stores, which applications use it, its strengths and limitations, and when you would want to convert from one to another. By the end, you will have a clear picture of the email format landscape and the knowledge to make informed decisions about your email data.
PST — Personal Storage Table
What Is a PST File?
PST stands for Personal Storage Table. It is the primary file format used by Microsoft Outlook on Windows to store a local copy of your mailbox data. A single PST file can contain emails, contacts, calendar events, tasks, notes, and the entire folder hierarchy you have set up in Outlook.
The format follows the Microsoft Open Specification (MS-PST) and uses a B-tree based internal structure. Despite the specification being public, the format is complex enough that few tools outside of Microsoft’s ecosystem can handle it natively.
What PST Files Contain
- Complete email messages with headers, body text, and attachments
- Contact records with all fields (name, phone, address, company)
- Calendar entries including recurring events and meeting metadata
- Task items with due dates, priorities, and status
- Notes and journal entries
- Folder structure and subfolder hierarchy
When You Encounter PST Files
PST files show up in corporate migrations, employee offboarding, legal discovery, and personal email backups. They are the default export format for Microsoft Outlook and the standard archive format in Windows-based organizations.
Limitations of PST
Older versions of PST (ANSI format) had a 2 GB file size limit. The newer Unicode format supports files up to 50 GB, though performance degrades with very large files. PST files require Outlook or a specialized tool to open, making cross-platform access a challenge.
If you need to work with PST files on a Mac or Linux machine, you will likely need to convert PST to EML or convert PST to MBOX first.
EML — Email Message
What Is an EML File?
EML is a plain-text file format for individual email messages. Each EML file represents a single email, stored in standard MIME format (RFC 5322). The format is human-readable if you open it in a text editor, though encoded attachments will appear as Base64 strings.
Compatibility
EML is the closest thing to a universal email format. It is natively supported by:
- Mozilla Thunderbird
- Windows Mail and Windows Live Mail
- Apple Mail (with some caveats)
- Most webmail platforms for import/export
- Nearly every email migration tool on the market
Structure
An EML file contains the complete email headers (From, To, Subject, Date, Message-ID), the body in plain text and/or HTML, and any attachments encoded inline. The format preserves the original message exactly as it was sent or received.
When to Use EML
EML is the go-to format when you need maximum portability. If you are moving emails between different clients, archiving individual messages, or need to inspect email headers for troubleshooting, EML is the right choice.
You can convert PST to EML to make Outlook archives accessible in virtually any email client, or convert EML to PST when importing messages into Outlook.
MBOX — Mailbox Format
What Is an MBOX File?
MBOX is one of the oldest email storage formats, dating back to the early days of Unix. It stores multiple email messages in a single file, with each message separated by a “From ” line (a line starting with the word “From” followed by a space).
Variants
Several variants of MBOX exist:
- mboxrd — Used by most modern applications, escapes “From ” lines in message bodies
- mboxo — The original format, does not escape “From ” lines (can cause parsing issues)
- mboxcl — Adds a Content-Length header to avoid ambiguity
- mboxcl2 — Combines Content-Length with escaping
In practice, most tools today produce and consume the mboxrd variant.
Where MBOX Is Used
- Mozilla Thunderbird (primary storage format)
- Apple Mail (historically)
- Gmail exports via Google Takeout
- Unix/Linux mail systems
- Many open-source email tools
Strengths and Weaknesses
MBOX files are simple, widely supported, and easy to work with programmatically. However, the single-file approach means you cannot access individual messages without parsing the entire file, and concurrent access can cause corruption.
For migration from Thunderbird to Outlook, you would convert MBOX to PST. Going the other direction, PST to MBOX conversion opens up Outlook archives for use in Thunderbird and other open-source clients.
MSG — Outlook Message
What Is an MSG File?
MSG is a Microsoft proprietary format for individual email messages. Unlike EML, which stores messages in plain MIME text, MSG uses the OLE (Object Linking and Embedding) Compound File Binary format — the same container technology used by old Word .doc and Excel .xls files.
What MSG Contains
Each MSG file stores a single email with:
- All standard email headers
- Message body in RTF, HTML, or plain text
- File attachments as embedded OLE objects
- Outlook-specific properties (categories, flags, follow-up dates)
- Sender and recipient MAPI properties
When You Encounter MSG Files
MSG files are typically generated when someone drags an email from Outlook onto their desktop or saves an individual message. They are common in legal proceedings, where individual emails need to be preserved as evidence, and in business workflows where emails are shared as file attachments.
Limitations
MSG files are tightly coupled to the Microsoft ecosystem. They require Outlook or a specialized viewer to open properly. The OLE compound file structure makes them harder to parse than EML files, and many non-Microsoft tools offer incomplete MSG support.
You can convert MSG to EML for universal compatibility or convert MSG to PST to consolidate individual messages into an Outlook-compatible archive.
EMLX — Apple Mail Message
What Is an EMLX File?
EMLX is Apple’s proprietary email file format used by Apple Mail (Mail.app) on macOS. Like EML, each EMLX file represents a single email message, but Apple adds a byte count header and an XML property list (plist) footer containing metadata like read/unread status and flags.
Structure
An EMLX file has three parts:
- A first line containing the byte count of the message content
- The standard RFC 5322 email message (same as EML)
- An XML plist section with Apple Mail metadata (message flags, date received, etc.)
Where EMLX Is Used
EMLX is used exclusively by Apple Mail on macOS. When Apple Mail downloads your email, it stores each message as a separate EMLX file within the ~/Library/Mail/ directory.
Working with EMLX Files
Because EMLX files are essentially EML files with Apple-specific wrappers, conversion is straightforward. You can convert EMLX to EML by stripping the byte count and plist data, or convert EMLX to PST to import Apple Mail messages into Outlook.
OST — Offline Storage Table
What Is an OST File?
OST stands for Offline Storage Table. It is essentially the offline counterpart to a PST file, created automatically by Outlook when you connect to an Exchange Server, Office 365, or IMAP account. The OST file allows you to work with your email while disconnected from the network; changes sync back to the server when you reconnect.
How OST Differs from PST
While OST and PST share a similar internal structure, there is a critical difference: OST files are tied to the specific Outlook profile and Exchange account that created them. You cannot simply copy an OST file to another computer and open it in Outlook. If the original account is deleted or the profile is corrupted, the OST file becomes an orphan — the data is there, but Outlook will not mount it.
When OST Conversion Is Necessary
OST conversion becomes essential when:
- An employee leaves and their Exchange account is deactivated
- An Exchange server is decommissioned
- The Outlook profile is corrupted
- You need to access cached data from an unavailable account
In all these cases, converting OST to PST is the standard approach to recover the data. You can also convert OST to EML or convert OST to MBOX for cross-platform access.
OLM — Outlook for Mac Archive
What Is an OLM File?
OLM is the export format used by Outlook for Mac. When you archive or export your mailbox from Outlook on macOS, it creates an OLM file rather than a PST file. OLM files use a ZIP-based container that holds individual email messages in XML or EML-like format along with metadata.
OLM vs. PST
Despite both being Microsoft Outlook formats, OLM and PST are completely different at the binary level. PST uses a B-tree database structure while OLM uses a compressed archive approach. You cannot open an OLM file in Outlook for Windows, and you cannot open a PST file natively in older versions of Outlook for Mac.
When You Need OLM Conversion
OLM conversion is necessary when:
- Moving from Mac to Windows (or vice versa)
- Switching from Outlook for Mac to another email client
- Consolidating Mac and Windows email archives
You can convert OLM to PST to make Mac Outlook archives accessible on Windows, or convert to other formats for broader compatibility.
Comparing All Email Formats at a Glance
| Feature | PST | EML | MBOX | MSG | EMLX | OST | OLM |
|---|---|---|---|---|---|---|---|
| Developer | Microsoft | IETF Standard | Unix tradition | Microsoft | Apple | Microsoft | Microsoft |
| Messages per file | Multiple | Single | Multiple | Single | Single | Multiple | Multiple |
| Structure | B-tree database | Plain MIME text | Concatenated text | OLE compound file | MIME + plist | B-tree database | ZIP archive |
| Primary client | Outlook (Windows) | Thunderbird, most clients | Thunderbird, Gmail | Outlook | Apple Mail | Outlook (cached) | Outlook (Mac) |
| Cross-platform | Limited | Excellent | Good | Limited | macOS only | None | macOS only |
| Human-readable | No | Yes | Partially | No | Partially | No | No |
| Attachments | Embedded in DB | Base64 inline | Base64 inline | OLE objects | Base64 inline | Embedded in DB | Embedded in archive |
| Max file size | 50 GB (Unicode) | ~5 MB typical | No hard limit | ~20 MB typical | ~5 MB typical | 50 GB | Varies |
How to Choose the Right Format
For Archiving
If you are archiving emails for long-term storage, EML offers the best combination of simplicity and portability. Each email is a standalone file that can be read by virtually any email client or even a text editor. For large-scale archiving, MBOX consolidates messages into fewer files, reducing filesystem overhead.
For Migration
The right format depends on your destination:
- Moving to Outlook: Convert to PST using MBOX to PST or EML to PST
- Moving to Thunderbird: Convert to MBOX using PST to MBOX
- Moving to any client: Convert to EML using PST to EML for maximum compatibility
- Moving to cloud: EML or MBOX are the most widely accepted import formats
For Legal and Compliance
Legal teams typically work with PST or EML formats. PST preserves the complete mailbox structure, while EML allows individual messages to be tagged, reviewed, and produced as separate exhibits. MSG files are also common in legal contexts because they preserve Outlook-specific metadata.
For Developers
EML is the most developer-friendly format — it is plain text, standards-based, and easy to parse programmatically. MBOX is also straightforward for batch processing. PST and OST require specialized libraries due to their binary structure.
Converting Between Email Formats
Why Conversion Is Necessary
No single email format works everywhere. Microsoft, Apple, Google, and open-source projects all use different formats, and switching between platforms means converting your data. The good news is that modern conversion tools handle the heavy lifting, preserving folder structure, attachments, metadata, and message integrity.
What to Preserve During Conversion
A good conversion should maintain:
- Message headers — From, To, CC, BCC, Date, Subject, Message-ID
- Body content — Both plain text and HTML versions
- Attachments — All attached files with original filenames
- Folder structure — The hierarchy of folders and subfolders
- Metadata — Read/unread status, flags, categories
- Character encoding — Correct handling of international characters
Online vs. Desktop Conversion
Online conversion tools like MailtoPst process your files on secure servers without requiring software installation. This is convenient for one-time conversions and works on any device. Desktop tools require installation but keep your data entirely local, which some organizations prefer.
MailtoPst operates on EU-based servers with automatic file deletion after 24 hours, addressing both convenience and privacy concerns. All conversions are GDPR compliant.
Common Conversion Paths
The most frequent conversion scenarios include:
- PST to EML — Opening Outlook archives in other clients
- PST to MBOX — Migrating from Outlook to Thunderbird
- MBOX to PST — Moving Thunderbird or Gmail data to Outlook
- EML to PST — Consolidating individual emails into Outlook
- OST to PST — Recovering data from orphaned OST files
- MSG to EML — Making Outlook messages universally readable
- OLM to PST — Migrating from Outlook for Mac to Windows
Security and Privacy Considerations
Data Protection During Conversion
Email files often contain sensitive personal and business data. When choosing a conversion tool, look for:
- GDPR compliance — Particularly important for European organizations
- Server location — EU-based servers are subject to stricter privacy regulations
- Automatic deletion — Files should be removed after conversion is complete
- Encryption in transit — HTTPS/TLS for all file transfers
- No data mining — The service should not read or analyze your email content
Local vs. Cloud Processing
If your organization handles classified or highly sensitive data, local processing may be required by policy. For most users, a reputable cloud-based tool with proper security measures offers the best balance of convenience and protection.
MailtoPst processes all conversions on GDPR-compliant EU servers with automatic deletion after 24 hours. No email content is ever read, stored, or shared beyond the conversion process.
Troubleshooting Format Issues
Corrupted Files
All email formats can suffer from corruption due to incomplete writes, disk errors, or improper shutdowns. PST files are particularly vulnerable because of their database structure — a corrupted B-tree node can make an entire branch of folders inaccessible. Microsoft provides the Inbox Repair Tool (scanpst.exe) for PST files, but third-party tools often do a better job with severe corruption.
Encoding Problems
International characters (accents, CJK characters, Cyrillic, Arabic) can cause issues during conversion if the tool does not handle character encoding properly. Always verify that your conversion tool supports UTF-8 and the specific encodings used in your email data.
Large File Handling
PST and MBOX files can grow to tens of gigabytes. Not all conversion tools handle large files gracefully — some run out of memory or produce incomplete output. For enterprise-scale conversions, look for tools that use streaming or chunked processing rather than loading entire files into memory.
Frequently Asked Questions
What is the most universal email file format?
EML is the most widely supported email format. It is based on the IETF RFC 5322 standard and can be opened by virtually every email client, including Thunderbird, Windows Mail, and most webmail services. If you need to ensure your emails can be read anywhere, converting to EML is the safest choice.
Can I open a PST file without Microsoft Outlook?
Yes. While PST is a Microsoft proprietary format, several tools can read PST files without Outlook. You can also convert PST to EML or convert PST to MBOX using MailtoPst, which works entirely in your browser without any software installation.
What is the difference between PST and OST files?
Both PST and OST use a similar internal structure, but they serve different purposes. PST files are standalone archives that can be moved between computers. OST files are cache files tied to a specific Outlook profile and Exchange/Office 365 account. If the account is removed, the OST file cannot be opened directly — you need to convert it to PST to recover the data.
Is MBOX or EML better for email archiving?
It depends on your needs. EML stores one message per file, making it easy to search, sort, and access individual emails. MBOX stores multiple messages per file, which is more efficient for large archives but requires parsing to access individual messages. For long-term archiving with easy access, EML is generally preferred. For compact storage, MBOX is better.
How do I convert Gmail emails to Outlook format?
Gmail exports via Google Takeout produce MBOX files. To import these into Outlook, you need to convert MBOX to PST. MailtoPst handles this conversion online — upload your MBOX file, and download the resulting PST file ready for import into Outlook.
Are email conversions safe? Will I lose data?
A well-designed conversion tool preserves all message content, headers, attachments, and folder structure. MailtoPst maintains data integrity throughout the conversion process and operates on GDPR-compliant EU servers with automatic file deletion after 24 hours. Always verify a sample of converted messages to confirm everything transferred correctly.
What format does Apple Mail use?
Apple Mail stores individual messages as EMLX files in the ~/Library/Mail/ directory. EMLX is similar to EML but includes Apple-specific metadata. You can convert EMLX to EML for cross-platform use or convert EMLX to PST for import into Outlook.