Use Enhanced Letterheads to Increase the Effectiveness of Lightning Email Templates
Where: This change applies to Lightning Experience in Essentials, Professional, Enterprise, Performance, Unlimited, and Developer editions.
Who: To create and edit enhanced letterheads, you need the Modify All Data permission or administrator rights. Anyone can use enhanced templates.
How: To add content, style, and images to your letterhead, open the App Launcher, and select Enhanced Letterheads.