Like a subject line, the preheader text appears right below the subject in an inbox. Think of it as an extension or secondary part of the subject line. It should hint at the content of the email without giving it away, but still encourage a user to open the email.

Some other possible functions of the preheader:

Give an overview of the content
Elaborate on the subject line
Tease an incentive

Length

The ideal preheader length is between 85 and 100 characters. It's has a little more room than the subject line depending on the email client.

Things to Avoid

Don't restate the subject line
Don't make it completely unrelated to the subject line

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