Hi @CS ,
but I can't find a way to make it work on Edge Chromium (we have Exchange 2016 on premise)
According to the official document below, "S/MIME in Outlook on the web in Exchange 2016 is only supported in Internet Explorer". So seems it's the expected behavior that S/MIME extension doesn't work on Edge Chromium.
S/MIME settings for Outlook on the web in Exchange Server
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