Last Updated: 30 Jul, 2025
What are Email File Formats?
An email file format refers to the specific structure or type of file used to store email messages. These formats dictate how email content is encoded, how attachments are handled, and how metadata such as sender, recipient, subject line, and timestamps are preserved. These formats ensure compatibility across different email clients, servers, and applications. Whether you’re backing up important emails, migrating to a new service, or sharing messages, understanding email file formats1 is essential.
Common Email File Formats
The most popular email file formats are EML, MSG, MBOX, and PST. Choosing the right one depends on which email client you use and whether you need to save single messages or entire mailboxes. Choosing the wrong format could lead to lost data or compatibility issues. It is also possible to programmatically convert these email file formats using email processing APIs and libraries. Below, we explore the most popular email file formats and how to pick the best one for your needs.
EML (.eml)
EML stands for Electronic Mail Format. Think of EML as the universal standard for a single email message. It’s a plain text file that stores the email’s content, headers (like “To,” “From,” “Subject”), and any attachments in a single package.
Used by: Thunderbird, Windows Mail, Apple Mail, Outlook Express
Pros:
- Lightweight, text-based format.
- Retains headers, content, and attachments.
- Works with most email clients.
Cons:
- Lightweight, text-based format.
- Retains headers, content, and attachments.
- Works with most email clients.
MSG (.msg)
MSG stands for Microsoft Outlook Message Format and is Microsoft Outlook’s proprietary format for a single item. An MSG file can be an email, an appointment, a contact, or a task. It preserves all the details and formatting exactly as they appear in Outlook. However, its biggest drawback is its limited compatibility; it’s designed to work best within the Microsoft ecosystem.
Used by: Microsoft Outlook.
Pros:
- Preserves formatting, attachments, and metadata.
- Supports embedded objects (like calendar invites).
Cons:
- Only fully compatible with Outlook.
MBOX (.mbox):
The MBOX format is a classic. Instead of saving one email per file, it concatenates multiple emails into a single, large text file. It’s the standard format used by many open-source and Unix-based email clients like Thunderbird and Apple Mail. It’s excellent for exporting and backing up entire email folders or mailboxes in one go.
Used by: Mozilla Thunderbird, Apple Mail, Eudora, Postbox.
Pros:
- Open standard, widely supported.
- Great for archiving or transferring email folders
- Stores multiple emails in a single file.
Cons:
- No built-in folder organization.
- Can become large and harder to manage.
PST (.pst)
The Personal Storage Table, or PST, is another Microsoft creation. It’s a database file that Outlook uses to store entire mailboxes—including emails, calendars, contacts, and tasks—for offline access. PST files are perfect for creating comprehensive backups of your Outlook data or for moving your entire account to a new computer. They can be password-protected for added security.
Used by: Microsoft Outlook.
Pros:
- Stores emails, contacts, calendars, and tasks in one file.
- Supports password protection.
Cons:
- Large file sizes.
- Limited compatibility outside Outlook.
OST (.ost)
An OST (Offline Storage Table) file is a data file used by Microsoft Outlook to store a synchronized, cached copy of your mailbox information on your computer. It’s automatically created when you set up an account that connects to a server, such as a Microsoft Exchange, Office 365, or IMAP account. The primary purpose of an OST file is to allow you to work offline. It enables you to read, compose, delete, and reply to emails, as well as manage calendar appointments and contacts, even when you don’t have an active internet connection.
Used by: Microsoft Outlook (with Exchange or Microsoft 365)
Pros:
- Allows working without internet.
- Syncs when reconnected.
Cons:
- Tied to specific accounts
- Cannot be directly opened in other email clients.
Final Thoughts
Selecting the right email file format depends on your email client, storage needs, and whether you plan to migrate or share emails. For individual messages, EML is versatile, while PST and MBOX are better for bulk storage. Outlook users should stick with **MSG and PSTv, whereas Apple Mail users will prefer EMLX. Always consider your use case, software compatibility, and long-term accessibility before settling on a file type. With the right email file format, managing your digital communications becomes more secure, efficient, and future-proof.
FAQ
Q: What is the best email file type for sharing a single message?
A: EML or MSG files are ideal for sharing individual email messages with attachments.
Q: Which file type is best for backing up an entire mailbox?
A: PST (for Outlook) or MBOX (for open-source clients) are best for full mailbox backups.
Q: Can I open MSG files without Microsoft Outlook?
A: MSG files are proprietary to Outlook, but some third-party tools and viewers can open them.