To send encrypted email through Outlook first required obtaining your own digital ID certificate from a valid provider. Review the link below from Microsoft about getting your own digital ID certificate. The section at the top about getting a digital ID has links to providers where you can purchase an ID:

https://support.microsoft.com/en-us/office/get-a-digital-id-0eaa0ab9-b8a2-4a7e-828b-9bded6370b7b

Once you have a digital certificate follow the steps to install the certificate into Outlook.
 

To encrypt an outgoing message follow the steps in this link below. Follow the S/MIME encryption options.

https://support.microsoft.com/en-us/office/encrypt-email-messages-373339cb-bf1a-4509-b296-802a39d801dc

The link has options for newer versions of Outlook as well as Office 2010 and Office 2007 versions.

You can send individual emails encrypted or set Outlook to send all messages encrypted.