When you open a captured message, the HTML tab renders it exactly as a recipient would see it, in an isolated frame. It is the fastest way to confirm that images, layout, buttons and links look right before an email ever goes out for real.

Alongside the preview you can inspect the source, headers and raw message, run the HTML & CSS compatibility check, and check the spam score.

Emails are read everywhere, so the preview lets you switch the viewport width to check your responsive design:

• Fit — scales the message to fit the available space
• Tablet — renders at a tablet-sized width
• Mobile — renders at a narrow phone width, where most email is opened

Switching widths triggers your media queries, so you can catch layouts that break on small screens before your client does.