Microsoft is investigating a bug preventing recipients from opening encrypted emails in classic Outlook after a recent update.
This known issue affects users who try to open messages encrypted with "Encrypt Only" permissions, which also allows copying, printing, and forwarding.
"After updating to Current Channel Version 2511 (Build 19426.20218) recipients may not be able to open 'Encrypt Only' emails," Microsoft said.
"In the Reading Pane you may get a message 'This message with restricted permission cannot be viewed in the reading pane until you verify your credentials. Open the item to read its contents and verify your credentials'."
On impacted systems, users are seeing a message_v2.rpmsg attachment instead of readable content, making the encrypted messages inaccessible.
Microsoft said that the Outlook Team is currently working on a fix, but did not provide an estimated timeline for resolution.
Encrypted email bug in classic Outlook (Microsoft)
Until a permanent fix is available, Microsoft provides two temporary workarounds. One of them requires senders to save encrypted emails after encryption and before sending, enabling recipients to open them normally.
Alternatively, users can revert to the previous software build that isn't affected by this known issue. This requires affected users to close all Office apps and run the following command from an elevated command prompt:
"%programfiles%\Common Files\Microsoft Shared\ClickToRun\officec2rclient.exe" /update user updatetoversion=16.0.19426.20186
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