What is the RPMSG file format?
.rpmsg files are email messages with restricted access, associated with the Microsoft Outlook application. Such files contain messages with confidential data and additional attachments whose content can be displayed only by a defined recipient.
.rpmsg files are used for control of access to messages, utilizing encryption and Information Rights Management (IRM). The message sender can also limit certain actions, like saving, printing, forwarding, or copying. Sending email messages is an easy and very popular method of modern communication; however, such messages may be a potential target for attacks by unauthorized third parties. This is why the .rpmsg format was created - to ensure the security of electronic messages. Introduced alongside Windows Rights Management Services and Outlook 2003 IRM, it is today part of Microsoft Purview Message Encryption.
Technically, .rpmsg is a binary container: content and attachments are packed into an OLE Compound File structure (the same binary chassis that underlies .msg files), then compressed with zlib/DEFLATE and prepended with a proprietary 4-byte signature (76 E8 04 60). The resulting archive is sent as an attachment - typically named message.rpmsg - appended to a standard .eml wrapper.
Additional information about .rpmsg files:
.rpmsgfiles are sent as attachments to standard email messages.- Creating an
.rpmsgfile is a process of storing content and attachments in a single file, which is then compressed and encrypted so that amessage.rpmsgfile is produced. - The format is proprietary and not publicly documented by Microsoft; its internal structure is understood through reverse engineering by open-source parsers.
- Earlier documentation stated RPMSG files could only be opened with Microsoft Outlook on Windows. Recipients can now open them in Outlook for Windows, Outlook for Mac, Outlook for iOS, Outlook for Android, Outlook on the web, and the Microsoft Purview encrypted message portal.
Security & safety
RISK: MEDIUMA genuine message.rpmsg is harmless - it is encrypted corporate mail. The real risk is phishing: attackers imitate Microsoft "encrypted message / Read the message" notifications to steal credentials. Verify that the 'Read the message' link leads to a microsoft.com / office365.com domain before signing in, and be suspicious of unexpected protected messages from unknown senders. The rpmsg file itself cannot carry executable code that runs on opening.
Format details
in a nutshellPrograms that open RPMSG files
Windows1 app
Microsoft Outlook (classic / new Outlook)
Paid Open the original e-mail while signed in as the intended recipient; Outlook fetches the rights license and shows the message automatically. The rpmsg attachment never needs to be opened manually. 
macOS1 app
Microsoft Outlook for Mac
Paid Sign in with the recipient's Microsoft 365 account and open the message; IRM messages are supported natively. 
Android1 app
Outlook on the web (outlook.office.com / outlook.com)
Free Sign in with the recipient address - protected messages render in the browser without the attachment.
Microsoft Purview encrypted message portal
Free Click the 'Read the message' button in the e-mail body, then verify with a one-time passcode or Google/Microsoft sign-in. This is the only route for Gmail/Yahoo recipients. Technical details
deep spec| Container format | Binary; zlib/DEFLATE-compressed OLE Compound File (CFB) carrying the DRM-protected message body and attachments |
| File signature (magic bytes) | 76 E8 04 60 at offset 0, followed by zlib-compressed chunks; decompressed payload opens with the standard OLE header D0 CF 11 E0 |
| MIME type | application/x-microsoft-rpmsg-message |
| Compression | zlib/DEFLATE applied to the OLE Compound File payload before the file is written to disk |
| DRM technology | Windows Information Rights Management (IRM) / Azure Rights Management (Azure RMS), now unified under Microsoft Purview |
| Configurable permissions | Sender can restrict forwarding, printing, copying, and saving on a per-message basis |
| Expiry enforcement | Sender can embed an expiration date; decryption is denied server-side by the RMS/Purview infrastructure after the deadline passes |
| Delivery mechanism | Attached to a plain-text wrapper email; the attachment is conventionally named message.rpmsg |
| Recipient authentication | Recipient must authenticate against Microsoft RMS or Microsoft Purview to obtain a per-message decryption key |
| License payload | Embeds an IRM use license (UL); may reference a server-side publishing license (PL) for key distribution to authorized recipients |
| Format specification | Proprietary and undocumented by Microsoft; internal structure derived from open-source reverse-engineering efforts |
| Developer | Microsoft Corporation |
| Supported clients | Microsoft Outlook (Windows, Mac, iOS, Android), Outlook on the web, Microsoft Purview encrypted message portal |
| Related protected format | .pfile - Azure RMS generic file-protection wrapper that applies the same IRM infrastructure to arbitrary file types |
| Released | 2003 (Windows Rights Management Services / Outlook 2003 IRM); today part of Microsoft Purview Message Encryption |
| Specification | learn.microsoft.com |
RPMSG conversions
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