The email editor allows you to see the building blocks of your email, including placeholders and panels.
To see how your email will look to your recipients, from the editor:

• Click the Preview button (1).
• From the dropdown, select View Preview.

In the preview, use the toggle (2) to switch from a desktop to a mobile preview.

Admins can toggle between user previews using the switcher (3).

It's important to note that not all placeholders will change in the preview.