Preheader
The short text snippet shown in the inbox after the subject line, before the user opens the email.
The preheader (sometimes called the preview text) is the short text snippet shown in the inbox after the subject line, before the user opens the email. Most clients show 40–100 characters of preheader; mobile clients show less.
The preheader is the most underused real estate in email. Default behavior is for the client to pull the first line of body text as the preheader, which often results in "Having trouble viewing this email?" or a navigation menu appearing as the preview. Explicit preheaders (set via an invisible <span> at the top of the HTML or via your ESP's preheader field) avoid this.
Good preheaders work as a second sentence to the subject line — they don't repeat the subject; they extend it. "Your cart's still here" (subject) + "And so is the free shipping" (preheader) is a classic two-beat pattern. Treat the preheader as a coordinated layer with the subject line, not an afterthought.