OWA, S/MIME on EDGE in MacOS 15.6 Sequoia

Hi M Mosher,

Welcome to Microsoft Q&A!

Thank you for reaching out. I totally understand and feel your frustration with the ongoing S/MIME control failure in Microsoft Edge, especially after you have already installed the extension and configured the domain settings correctly.

This happens because the S/MIME extension relies on native messaging components that are only available on Windows platforms, which macOS normally does not support. Due to that, the extension cannot fully enable S/MIME functionality in Outlook on the Web (OWA), and the error message you’re seeing is a fallback placeholder indicating that the platform is unsupported.

You can find a similar issue discussed here: SMIME Extension Error in EDGE - Microsoft Q&A with a proven solution to enable S/MIME capabilities in OWA for supported platforms.

• If you're in a business environment, contact your admin/IT
• Run Set-SmimeConfig (ExchangePowerShell) | Microsoft Learn
• Enable the OWAIncludeSMIMECapabilitiesInMessage parameter.

  Set-SmimeConfig -OWAIncludeSMIMECapabilitiesInMessage $true

There are also other options you can try if the above solution doesn't work:

1 - Fix certificate trust in macOS

• Open Applications > Utilities > open Keychain Access
• Locate your S/MIME certificate under My Certificates
• Double-click > expand Trust section > set up:
  • When using this certificate: Always Trust
  • Secure S/MIME: Always Trust

2 - Enable experimental flags (if still not working)

• Open Edge > access edge://flags/#enable-smime
• Enable the following flags:
  • #enable-smime
  • #enable-experimental-web-platform-features

We look forward to hearing from you and reviewing the results. Please understand that our initial reply may not immediately resolve the issue. However, with your help and more detailed information, we can work together to find a solution. Thank you for your time.

Warm Regards,

Lychee-MSFT | Microsoft Q&A Support Specialist

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Hi M Mosher,Just checking in to see if the above information was helpful. If you have any further updates on this issue, please feel free to post back. If you think my reply is helpful to you, please remember to mark it as an answer. Warm thanks.

Lychee-MSFT | Microsoft Q&A Support Specialist

Lychee,

That solution did not work. First do not have the ability to modify the business environment, I contacted them about why no one can find a fix for this when it is a requirement to use.

I can not Enable the following flags: "#enable-smime"

That flag does not exist in the browser list, I did a search from the experiments page and a text search by "CMD-F"