How to use S/MIME certificate in Outlook on the web and desktop client

Hi MarcelThieme,

If you are using Exchange Online without an on-premises Active Directory (AD), you can still configure and use S/MIME email certificates for users. Here are the short steps:

1. Access the Exchange admin center (EAC):

   • Go to the Microsoft 365 admin center (admin.microsoft.com).
   • Select "Exchange" from the admin centers menu.
     
     

2. Configure S/MIME settings:

   • In the EAC, go to "Mail flow" > "Message encryption" > "S/MIME."
   • Enable S/MIME by checking the "Enable S/MIME signing and encryption" box.
   • Choose the desired options for signing and encryption.
   • Save your settings.
     
     

3. Upload the user's S/MIME certificate:

   • In the EAC, navigate to "Recipients" > "Mailboxes."
   • Select the user mailbox for which you want to upload the S/MIME certificate.
   • Click on "More options" (•••) in the mailbox details pane and choose "Manage S/MIME settings."
   • Use the "Import certificate" button to browse and locate the user's S/MIME certificate file.
   • If needed, enter the certificate password and click "OK" to upload the certificate.

Once the S/MIME certificate is uploaded, you should be able to use it with Outlook on the web, mobile, and desktop clients.