Outlook S/MIME Digital Signature Warning

Issue

User sees this message at the top of an email in the Outlook Web app:

This message has a digital signature, but it wasn't verified because the S/MIME control isn't currently supported for your browser or platform.

Environment

Managed MacOS Computer

Description

• The user was sent an email with an attachment and a self-signed digital signature, but the Outlook web app was throwing an error in both Chrome and Safari due to that signature.
• The attachment was able to be opened in the Outlook desktop app.

Resolution

• The S/MIME error refers to a protocol that only works on web clients when on Windows devices.
• It works on Mac desktop apps (Outlook for example), but the Outlook Web app for Mac devices does not support the protocol.
  • That is why the error is visible on both Chrome and Safari, but not on the Outlook desktop app.

Resources used

• Configure S/MIME in Exchange Online (Microsoft Learn)
• Encrypt messages by using S/MIME in Outlook on the web (Microsoft Support)