Microsoft: Classic Outlook bug prevents opening encrypted emails

If your team relies on secure communications, you can’t ignore the newly uncovered encrypted email bug in classic Outlook. A recent Microsoft security update to improve protection has unintentionally disrupted how users open encrypted emails. While Microsoft is working on a fix, the issue has already confused businesses that depend on email encryption to protect sensitive information.

Why You Need To Pay Attention to This Microsoft Bug
Encrypted emails aren’t just nice-to-haves. They’re often required for compliance, to protect trade secrets, or to maintain client trust.

When the Outlook app breaks email decryption like this, it creates real headaches: delayed responses, missed opportunities, and potential security risks if people start looking for workarounds outside of official channels. The bug doesn’t hit everyone, but if your team sends or receives “Encrypt Only” messages regularly, you’re probably already facing confusion.

What Triggered the Issue in Classic Outlook?
Microsoft posted an update on January 6, 2026, to explain the issue. They confirmed that the problem began after users installed Current Channel Version 2511 (Build 19426.20218). After updating, some recipients found they could no longer open “Encrypt Only” messages in classic Outlook. Instead, they received a message requesting that they verify their credentials.

Even after clicking to open the message and entering credentials, the email often appears as an unreadable attachment, such as message_v2.rpmsg, rather than the actual content. Unfortunately, opening the message doesn’t always solve the problem, because the issue is tied to how Outlook handles email decryption after the recent security update, not user credentials or permissions.

Two Workarounds
After opening the email, users will find an attachment message_v2.rpmsg and won’t be able to read the contents.  The problem happens for users who encrypt the email using the settings under the email File dialog (File > Encrypt), Microsoft further explained.

To prevent the issue, there are two possible workarounds. For the first one, users should select the Options ribbon, then select Encrypt, and choose Encrypt or Do Not Forward.

The second workaround is to revert to a build before the issue started and then send the email. The issue started with build 19426.20218, and the next prior build that works is 19426.20186. To revert Office, users should open an elevated Command Prompt, and run this command: “%programfiles%\Common Files\Microsoft Shared\ClickToRun\officec2rclient.exe” /update user updatetoversion=16.0.19426.20186

For organizations that exchange contracts, financial data, or internal credentials by email, this disruption can slow down daily operations.