Glossary

What Is an RPMSG File?

An RPMSG file is a rights-protected email message. Microsoft Information Rights Management (IRM) packs the body, images, and attachments into an encrypted attachment named message.rpmsg and attaches it to an ordinary email. Only the people the sender authorized can decrypt it. A file that refuses to open is intact; your account has no key to it.

TL;DR

RPMSG is an encrypted Outlook message, not a broken file. Sign in as the intended recipient to read it, then save the message as MSG if you need to convert or archive it.

Why an RPMSG File Refuses to Open

Double-click message.rpmsg on a machine without Outlook and you get nothing readable. The content is encrypted, and the rights service grants access by identity: the sender listed who may read the message, and that list is checked before anything is decrypted. Sign in as the intended recipient and the same file renders as an ordinary email. The file is fine. Your credentials are the problem.

What Is Inside a message.rpmsg File

The attachment is a sealed bundle, and a text editor shows nothing but binary noise. Three things go inside it:
  • The message body, images, and attachments, written into a compound file.
  • That container encrypted, then compressed with the DEFLATE algorithm into a single blob.
  • The sender's permission rules, such as do not forward, do not print, do not copy.

The blob usually arrives under the name message.rpmsg, with the media type application/x-microsoft-rpmsg-message; some Encrypt-Only messages come through as message_v2.rpmsg instead. The protection itself comes from Azure Information Protection, Azure Rights Management, or Active Directory Rights Management Services.

Protection stays with the file. Forwarding the mail, copying the attachment to a USB stick, or moving it to another mailbox changes nothing. The rules and the encryption live inside the blob.

How to Open an RPMSG File

Outlook does the work when the identity matches:
  • Sign in to Outlook with the exact mailbox the message was addressed to, rather than a personal or forwarded account.
  • Open the message and click the message.rpmsg attachment.
  • Outlook extracts and renders the protected message inside the reading pane, with the sender's restrictions applied.

Opening an RPMSG File Without Outlook

If the mail landed in Gmail or another client, click the attachment and you land on Microsoft's encrypted message portal in your browser. Sign in there with a Microsoft 365 account, with a Google or Yahoo identity, or with a one-time passcode sent to that same address. On Windows, the Microsoft Purview Information Protection viewer also lists .rpmsg among the protected file types it opens.

Treat an unexpected protected message with suspicion. In 2023, phishing crews used rpmsg attachments for that exact reason: the sign-in prompt looks legitimate, and the cloned Microsoft 365 form behind it harvests the password. Verify the sender through another channel before you type anything.

Can You Convert an RPMSG File to PDF?

Not directly. A tool that offers to decrypt one without your credentials cannot deliver. The workable route runs through Outlook. Read the message as the authorized recipient, save it as a regular MSG file when the sender's policy allows saving, then convert that copy. If the label blocks saving, printing, or exporting, there is no copy to convert, and the honest next step is to ask the sender for an unprotected version.

What you'll need
  • MSG to PDF Converter: batch-converts saved Outlook MSG messages to PDF on Windows
  • Outlook signed in with the account the message was sent to
  • Permission from the sender to save or export the message

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