Time needed: 30 minutes. These instructions detail how to install an S/MIME certificate and send secure email messages with Microsoft Outlook on Windows PCs. Testing was done in Outlook version 1902 on Windows 10 Enterprise, but Outlook has natively supported S/MIME for many years.
Launch Outlook. Choose the Tools menu and select Accounts. Under the list of accounts in the left pane of the Accounts window, click the plus sign (+) and select Exchange. In the Exchange account information dialog box enter the following: Email: Your PennO365 username (e.g., email@example.com).