How do I send an encrypted email from Outlook?

which Outlook, and is it a work account? that changes the answer. With a Microsoft 365 account there's an Encrypt button when you write a new message (Options tab in the desktop app, top of the compose window in the new Outlook / web). That uses Microsoft's own encryption and the person on the other end gets a link to read it even if they don't use Outlook. If your company hasn't turned it on, the button won't be there. The older S/MIME route needs a certificate installed for both you and the recipient, which is a pain for one-off emails.