IT STAFF / ADVANCED: Learn how to enable digital signatures with Outlook on the web, and how to sign and encrypt messages.
The browser being used for accessing Outlook on the web also must have the S/MIME extension installed. If it is not, you will see a message like one of the following:
The S/MIME extension (once installed) will not work for validating a signature (and may not work for signing/encrypting) unless the email is opened in a separate window (ie. pop-out). You may see a message to this effect:
If you receive an email that's signed and you do not have the extension, there will be a message informing you and offering a link to install the extension, this will take you to Edge Add-ons store
You may see this warning if you imported certificate, with strong private key protection:
It's not a bad idea to keep this warning, you can re-import the cert if you have a backup copy with private key included.
Just unselect the strong private key protection: