RPMSG File: The Basic Definition

An RPMSG file (short for Restricted Permission Message) is an encrypted email message created by Microsoft Outlook using Information Rights Management (IRM) technology. The file extension is .rpmsg, and these files most commonly appear as an attachment named message.rpmsg.

When a sender in Microsoft Outlook applies IRM protection to an email, the message is encrypted using Microsoft's Rights Management Services (RMS). Depending on your email client, you either see the protected message directly (if Outlook decrypts it transparently) or you receive it as an .rpmsg attachment that your email client couldn't open natively.

TL;DR: RPMSG = encrypted Outlook email. The encryption is controlled by Microsoft and tied to your identity. You need to authenticate with your Microsoft account to read the content.

What Is Microsoft Information Rights Management (IRM)?

Information Rights Management is an enterprise data protection feature built into Microsoft 365. It allows senders to control what recipients can do with an email — even after it has been delivered. IRM is powered by:

  • Azure Information Protection (AIP) — Microsoft's cloud-based rights management platform used by most modern Microsoft 365 tenants
  • Active Directory Rights Management Services (AD RMS) — the on-premise predecessor, still used in some enterprise environments

When IRM is applied to an email, the sender can enforce restrictions such as:

  • 🚫 No Forwarding — recipients can't forward the email to others
  • 🚫 No Copying — recipients can't copy text from the email body
  • 🚫 No Printing — the print function is disabled
  • ✅ Read-Only — recipients can only read the content
  • ⏱ Expiry Date — the email becomes inaccessible after a set date

Why Did You Receive a message.rpmsg File?

You receive a message.rpmsg attachment for one of these reasons:

  • Your email client doesn't support IRM natively. Gmail, Apple Mail, Thunderbird, and other non-Microsoft clients can't display IRM-protected content inline. So they show it as an attachment instead.
  • You forwarded an IRM-protected email. Some forwarding scenarios cause the IRM wrapper to become visible as an attachment at the destination.
  • Your Outlook configuration. Older or misconfigured Outlook installations may not handle IRM transparently and present the file as an attachment.
Security Note: Since 2023, hackers have been using .rpmsg files in sophisticated phishing attacks. If you received an unexpected RPMSG from an unknown sender claiming you need to "view a confidential message," be very cautious. Read our RPMSG phishing guide.

How RPMSG Files Work Technically

At the technical level, an RPMSG file is an OLE/CFB (Object Linking and Embedding / Compound File Binary) container — the same format used by old-style Microsoft Office files (.doc, .xls). Inside this container are several specialized streams:

  • EncryptedPermissionToken — Contains the XrML (eXtensible Rights Markup Language) license. This defines who can open the file and what they're allowed to do. The cryptographic keys are encrypted, but some metadata (issuer domain, template name, issue date) may be readable.
  • DRMTransform / Primary — The actual encrypted message content (body, attachments). This cannot be read without decryption keys from Microsoft's RMS servers.
  • Version — The version of the RPMSG format
  • \x01CompObj — OLE object class descriptor

When you open an RPMSG in Outlook, the client sends the EncryptedPermissionToken to Microsoft's RMS server, which verifies your identity, checks you're an authorized recipient, and returns decryption keys. Your Outlook then decrypts the content locally — the decrypted email never travels over the network.

How to Open an RPMSG File

There are several ways to access an RPMSG file's content:

  1. Outlook Web App — Log in to outlook.com with the exact email address the message was sent to. Open the original email — OWA handles IRM transparently.
  2. Microsoft OME Viewer — In Gmail or another client, look for a "Read the message" button. This opens Microsoft's Office Message Encryption viewer where you authenticate and see the content.
  3. Outlook Desktop — Open the Outlook app, make sure you're signed in with the right account, then double-click the .rpmsg file.
  4. Outlook Mobile App — Install the official Microsoft Outlook app on iPhone or Android and sign in.

For a full walkthrough of all methods, see our guide: How to Open RPMSG Without Outlook (5 Methods) →

Analyze Your RPMSG File Online

Want to understand what's inside your RPMSG file before opening it? Our free online RPMSG viewer parses the file structure entirely in your browser — zero upload, zero privacy risk. You'll see:

  • All internal OLE streams and their sizes
  • The protection type (AIP, IRM, AD RMS)
  • Issuer domain, content ID, and template name (when available in the token)
  • Hex dump of the file header
  • Personalized instructions for opening the content

Frequently Asked Questions

Is an RPMSG file dangerous?

Legitimate RPMSG files from known senders are safe — they're standard Microsoft business email files. However, cybercriminals have weaponized the format in phishing attacks because email scanners can't inspect encrypted content. Always verify unexpected RPMSG files with the sender before clicking any links inside them.

Can any tool actually decrypt an RPMSG file?

No legitimate third-party tool can decrypt RPMSG content. The encryption uses Microsoft's RMS infrastructure, which requires authentication with Microsoft's servers. Any website claiming to "decrypt" or "convert" RPMSG files is either misleading or poses a serious security risk.

What's the difference between RPMSG and MSG?

MSG files are standard Outlook email files — not encrypted, can be opened with many tools. RPMSG files are encrypted MSG files with rights management applied. See our RPMSG vs MSG vs EML comparison →