Configure S/MIME settings in Exchange Online for Outlook on the web
As an admin for Exchange Online, you can set up Outlook on the web (formerly known as Outlook Web App) to allow sending and receiving S/MIME-protected messages. Use the Get-SmimeConfig and Set-SmimeConfig cmdlets to view and manage this feature in Exchange Online PowerShell. To connect to Exchange Online PowerShell, see Connect to Exchange Online PowerShell.
Considerations for Chrome
To use S/MIME in Outlook on the web in the Google Chrome web browser, you (or another admin) must set and configure the Chromium policy named ExtensionInstallForcelist to install the Microsoft S/MIME extension in Chrome. The policy value is
maafgiompdekodanheihhgilkjchcakm;https://outlook.office.com/owa/SmimeCrxUpdate.ashx. And note that applying this policy requires domain-joined computers, so using S/MIME in Chrome effectively requires domain-joined computers.
For details about the ExtensionInstallForcelist policy, see ExtensionInstallForcelist.
This step is a prerequisite for using Chrome; it does not replace the S/MIME control that's installed by users. Users are prompted to download and install the S/MIME control in Outlook on the web during their first use of S/MIME. Or, users can proactively go to S/MIME in their Outlook on the web settings to get the download link for the control.
For more information