Understanding Email File Formats
If you work in IT, legal eDiscovery, or routinely manage email archives, you'll inevitably encounter two dominant file formats: MSG and EML.
At first glance, they seem to do the exact same thing—saving a single email message to your computer. However, beneath the surface, these formats are fundamentally different in structure, compatibility, and use cases. Understanding these differences will help you choose the right format for archiving and ensure you have the right tools to access them.
What is an EML File? (.eml)
EML (Electronic Mail Format) is an open-standard format developed in the early days of email (initially as RFC 822 and later updated). It is fundamentally a structured, plain-text file that contains the raw email data exactly as it was transmitted over the internet.
Key Characteristics of EML:
- Universal compatibility: Almost every major email client (Thunderbird, Apple Mail, Outlook Express, Windows Mail) can open EML files natively.
- Text-based: Because it's plain text, you can technically open an EML file in Notepad to read the raw headers and HTML source code.
- MIME structure: Attachments and embedded images are encoded directly within the text using Base64 encoding.
What is an MSG File? (.msg)
MSG (Outlook Item Format) is a proprietary, binary file format created by Microsoft specifically for Outlook and the Exchange ecosystem. Unlike EML, which only stores the transmitted message, MSG relies on Microsoft's Compound File Binary format to store a complex web of MAPI (Messaging Application Programming Interface) properties.
Key Characteristics of MSG:
- Outlook-centric: Designed specifically to be created and read by Microsoft Outlook.
- Complete fidelity: It captures *exactly* how the item looks and functions inside Outlook, including specific Outlook-only properties (like follow-up flags, categories, and voting buttons).
- Supports other item types: While commonly used for emails, the `.msg` extension is also used for Outlook Contacts, Calendar Appointments, and Tasks.
MSG vs. EML: Which is Better?
There is no definitive "better" format; it depends entirely on your environment and requirements.
Choose EML when:
- Cross-platform compatibility is essential. If you need to share emails with users on Macs, Linux machines, or mobile devices, EML is the safer bet.
- Long-term independent archiving. Because EML is an open standard and text-based, you can be confident that software will be able to read it decades from now.
- Using non-Microsoft clients. If your organization uses G Suite, Thunderbird, or Apple infrastructure, EML is the native choice.
Choose MSG when:
- You operate in a strict Microsoft environment. If everyone uses Outlook and Exchange, MSG provides the most accurate and seamless experience.
- Preserving Outlook features is critical. Legal teams and eDiscovery processes often require MSG files because they retain proprietary metadata (like read receipts, specific Exchange headers, and complex calendar event data) that might be stripped or translated incorrectly when converted to EML.
- Archiving appointments and contacts. EML only handles email messages, whereas MSG handles the full spectrum of Outlook items.
How to Open Both Formats
If you receive an EML file, you're usually in luck—the built-in Mail app on Windows and Mac will generally open it without fuss.
MSG files, however, are notoriously difficult to open without installing the full, paid version of Microsoft Outlook. If you receive an MSG file and don't have Office installed, Windows will throw a "How do you want to open this file?" error.
If you regularly need to access MSG files but don't want to purchase an Outlook subscription, we recommend using a dedicated, standalone viewer like OpenMSG. It instantly parses the proprietary binary structure of MSG files, allowing you to view the rich HTML content and attachment details natively on Windows, regardless of whether Outlook is installed.