RPMSG vs MSG vs EML: Key Differences Explained

Quick Comparison Table

RPMSG — Restricted Permission Message

An RPMSG file is an encrypted, rights-managed email created by Microsoft Outlook. It's the only email format here that is always encrypted — the content is locked with Microsoft's Rights Management Services and can only be decrypted after authenticating with Microsoft's servers.

When you see it: Corporate environments where senders want to restrict forwarding, printing, or copying of sensitive emails. Also used (and abused) in phishing attacks since 2023.

How to open:

• Outlook Web App (outlook.com) — recommended
• Microsoft OME Viewer — when emailed to Gmail users
• Microsoft Outlook desktop or mobile app
• Our free RPMSG viewer — for structure analysis and personalized guidance

Key technical facts: Uses OLE/CFB binary container format. Contains streams: EncryptedPermissionToken (XrML license), DRMTransform/Primary (encrypted body), Version.

MSG — Outlook Message File

An MSG file is Microsoft Outlook's standard email format for saving emails to disk. When you drag an email from Outlook to your desktop, it saves as a .msg file. MSG files are NOT encrypted — all email content (headers, body, attachments) is readable with the right tools.

When you see it: Email backups, archiving, legal discovery/eDiscovery, when someone exports or saves an Outlook email.

How to open:

• Microsoft Outlook (just double-click)
• Windows Mail
• Third-party tools: Thunderbird (with ImportExportTools NG plugin), various MSG viewer tools
• Online MSG viewers (safe since content is unencrypted)
• Python: extract-msg library for developers

Key technical facts: Also uses OLE/CFB binary container format — same as RPMSG, but without encryption. Contains MAPI (Messaging Application Programming Interface) properties. Fully parseable with open-source tools.

EML — Electronic Mail Format

An EML file is the universal, plain-text email format. It's based on the MIME (Multipurpose Internet Mail Extensions) standard used by virtually every email system on the planet. EML files are human-readable — you can open one in a text editor and see the headers and body (though attachments are base64 encoded).

When you see it: Email exports from Gmail, Apple Mail, Thunderbird, and most email clients. Also used for automated email processing, spam analysis, and email archiving.

How to open:

• Any email client: Gmail, Apple Mail, Outlook, Thunderbird, etc.
• Any text editor (for manual inspection)
• Numerous online EML viewers
• Browser extension EML viewers

Key technical facts: Plain ASCII/UTF-8 text format. Follows RFC 5322. Starts with email headers (From, To, Date, Subject, MIME-Version, Content-Type) followed by the message body. Attachments are base64 encoded inline. Some EML files use S/MIME signatures or encryption, but the EML format itself is plain text.

Which Format Do You Have?

If you're not sure which format you're dealing with:

• Look at the file extension: .rpmsg, .msg, or .eml
• The filename is often message.rpmsg (RPMSG) or email.eml (EML)
• Try opening in a plain text editor: if readable headers appear, it's EML. If you see binary gibberish starting with ÐÏ, it's MSG or RPMSG
• Use our RPMSG viewer — it validates whether a file is a valid OLE/CFB container and tells you if it's rights-protected

Summary

Think of it this way:

• EML = standard email. Open anywhere. No restrictions.
• MSG = Outlook email saved to disk. Microsoft format, widely supported. No encryption.
• RPMSG = encrypted MSG. Microsoft IRM-locked. Requires Microsoft authentication. Limited to authorized recipients.

If you need to open an RPMSG file, see our complete guide: How to Open RPMSG Without Outlook →