The email preheader is the short snippet of text that appears next to an email’s subject line in a user’s inbox. The preheader is intended to provide a brief preview or summary of the email content to give the recipient more context before opening the message. For email marketers, preheaders provide a valuable opportunity to enhance an email's subject line and entice recipients to open the message.
Although writing a preheader is not required to send an email, doing so is considered best practice for commercial email and typically increases email open and engagement rates.