Quick Comparison
| Concept | OME | RPMSG |
|---|---|---|
| What it is | Microsoft encryption product / packaging flow | File format for the protected message payload |
| You typically see | HTML “Read the message” wrapper in Gmail/Yahoo | message.rpmsg attachment |
| Requires Microsoft auth? | Yes | Yes (to decrypt content) |
| Can third parties fully decrypt offline? | No | No |
| Safe analysis without decrypting | Inspect wrapper + links carefully | Parse structure with RPMSG Viewer |
| Phishing risk | Fake OME pages steal credentials | Unexpected .rpmsg used as lure |
How they fit together
Office Message Encryption (OME) is Microsoft’s way to send encrypted email to people inside and outside the organization. When the recipient uses Gmail or another non-Outlook client, Microsoft often delivers a friendly HTML message with a button that opens the Office Message Encryption viewer after sign-in.
Under the hood, the protected content is frequently carried as an RPMSG (Restricted Permission Message) object — a rights-managed, encrypted binary package. That is why you may only “see” message.rpmsg when something in the OME wrapper fails, when the mail is forwarded oddly, or when a client surfaces the attachment instead of the button.
How to open protected mail safely
- Prefer the official path: use the “Read the message” link / Outlook on the web with the exact recipient account.
- Do not upload RPMSG files to random “online decrypt” sites — they cannot legitimately decrypt IRM without Microsoft and may steal files.
- If you only have the attachment, use the free RPMSG Viewer to understand structure and protection type, then open via Microsoft-supported methods. Full walkthrough: Open RPMSG without Outlook.
- Treat unexpected encrypted mail as high risk — see RPMSG phishing guide.
Recommendation
Think of OME as the delivery experience and RPMSG as the protected payload format. For end users: follow Microsoft’s viewer when available. For IT and security: educate users on lookalike OME pages and verify unexpected RPMSG with structure analysis before clicking embedded links.
More format context: RPMSG vs MSG vs EML · What is an RPMSG file?