Litmus

Litmus solves a problem every newsletter writer eventually runs into: an email that looks perfect in your own inbox can render broken in someone else's. It generates previews of how your email will look across dozens of email clients and devices — Outlook, Gmail, Apple Mail, and more — before you send.

That matters more than it might seem, because email clients each handle HTML and CSS differently, and a layout that breaks in Outlook can mean a meaningful chunk of your list sees a garbled version of your issue. Running a check before sending catches that ahead of time rather than after a batch of readers reply asking what happened.