MSG vs EML: What's the Difference and How to Convert

Two Formats, One Purpose

If you have ever dealt with email archives or migrated messages between platforms, you have probably encountered two common file extensions: .msg and .eml. Both store individual email messages, but they do it in fundamentally different ways. Understanding those differences is essential when choosing the right format for archiving, sharing, or migrating your email data.

What Is the MSG Format?

MSG is the native single-message format used by Microsoft Outlook. Each .msg file contains one email message — including its body, headers, attachments, and rich metadata — stored inside an OLE2 compound document. OLE2 (Object Linking and Embedding) is a binary container format developed by Microsoft that functions like a mini file system, with internal streams and storage objects.

Because MSG files are built on this proprietary structure, they carry a wealth of Outlook-specific information. This includes extended MAPI properties such as message flags, follow-up status, categories, sensitivity labels, sender account details, and conversation threading data. All of this metadata is preserved natively within the binary container.

The trade-off is compatibility. MSG files are designed to work within the Outlook ecosystem. Opening them outside of Outlook requires specialized parsing libraries that understand the OLE2 structure and MAPI property schema.

What Is the EML Format?

EML is a plain-text email format based on RFC 5322, the internet standard that defines how email messages are structured. An .eml file is essentially the raw source of an email as it travels across the internet — headers at the top, followed by the MIME-encoded body and attachments.

Because EML follows an open, well-documented standard, it is supported by virtually every email client across every platform. Thunderbird, Apple Mail, Windows Mail, and most webmail services can open EML files natively. You can even open an EML file in a text editor and read its contents directly.

EML files use MIME (Multipurpose Internet Mail Extensions) to encode attachments, inline images, and alternative content representations like HTML and plain text. This makes them highly portable and straightforward to process with standard tools.

Key Differences at a Glance

Feature	MSG	EML
Underlying format	OLE2 binary compound document	Plain text (RFC 5322 / MIME)
Openness	Proprietary (Microsoft)	Open internet standard
Platform support	Primarily Outlook / Windows	Cross-platform, nearly universal
Metadata richness	Extensive (MAPI properties, flags, categories)	Standard email headers
Readability	Binary — requires specialized tools	Human-readable in any text editor
Typical file size	Slightly larger due to OLE2 overhead	Generally more compact
Attachment handling	Embedded in OLE2 streams	MIME-encoded (Base64)

When MSG Is the Better Choice

MSG files excel in environments where Outlook is the primary email client and preserving every piece of Outlook-specific metadata matters.

• Enterprise Outlook environments — If your entire organization runs on Outlook and Exchange, MSG files retain all the native properties that Outlook relies on: categories, follow-up flags, voting buttons, sensitivity labels, and more. Moving emails between Outlook installations as MSG files ensures nothing is lost in translation.
• Legal and compliance archives — In e-discovery scenarios, the completeness of MSG files can be valuable. Extended MAPI properties may contain information relevant to legal proceedings that would not survive a conversion to a simpler format.
• Outlook-specific workflows — If you rely on Outlook features like custom forms, message rules based on MAPI properties, or deep integration with other Microsoft products, keeping messages in MSG format preserves that functionality.

When EML Is the Better Choice

EML files are the go-to format when portability, simplicity, and broad compatibility are priorities.

• Cross-platform sharing — If recipients might open emails on Mac, Linux, mobile devices, or web-based clients, EML is the safest choice. There is no dependency on any single vendor’s software.
• Long-term archiving — Because EML is based on an open internet standard, it is far more future-proof than a proprietary binary format. See our email archiving best practices guide for more on choosing the right format for long-term storage. Archives stored as EML files can be read decades from now with minimal tooling.
• Non-Outlook environments — If your team uses Thunderbird, Apple Mail, or any other client, EML files integrate seamlessly. There is no risk of metadata being misinterpreted or lost due to format incompatibility.
• Automated processing — Developers and system administrators often prefer EML because it can be parsed with standard MIME libraries in any programming language. No OLE2 parsing libraries are needed.

How to Convert Between MSG and EML

Whether you are moving from an Outlook-centric environment to a cross-platform setup, or consolidating archives into a single format, converting between MSG and EML is straightforward with MailtoPst.

Converting MSG to EML

1. Upload your MSG files to MailtoPst directly from your browser.
2. Preview each message to verify its content, headers, and attachments.
3. Select EML as the output format and start the conversion.
4. Download your converted EML files, ready to use in any email client.

Converting EML to MSG

The process works in reverse just as easily:

1. Upload your EML files to MailtoPst.
2. Preview the messages in the web interface.
3. Select MSG as the output format.
4. Download the MSG files for use in Outlook.

Because MailtoPst is the only cloud-based email converter, the entire process runs in your browser. There is no software to install, no plugins to configure, and it works on any operating system.

Batch Conversion for Large Collections

When dealing with hundreds or thousands of email files, converting them one at a time is not practical. MailtoPst supports batch conversion — you can upload an entire folder of MSG or EML files and convert them all in a single operation. The folder structure and file organization are preserved throughout the process, so you do not end up with a disorganized pile of converted files.

This is especially useful during email migrations, when an IT team needs to convert an entire department’s archived messages from one format to another on a tight deadline.

Metadata Preservation in Both Directions

One of the most common concerns during email format conversion is data loss. MailtoPst is designed to preserve all available metadata in both directions:

• MSG to EML — Standard email headers (From, To, Cc, Bcc, Date, Subject), the full message body (HTML and plain text), all attachments, and inline images are transferred intact.
• EML to MSG — The same core data is preserved, and the resulting MSG files are fully compatible with Outlook’s internal property structure.

No email content, attachment, or header information is silently dropped during conversion.

Choose the Right Format for Your Needs

The decision between MSG and EML ultimately comes down to your environment and goals. If you work exclusively within the Outlook ecosystem and need maximum metadata fidelity, MSG is the natural choice. If you need portability, openness, and compatibility across platforms, EML is the clear winner.

And when you need to move between the two, MailtoPst handles the conversion cleanly — preserving your data, your attachments, and your folder structure every step of the way.