What Is Preheader Text?
TL;DR
Preheader text is the preview snippet an inbox shows beside or beneath the subject line, taken from the first readable text in the email body unless the sender sets it explicitly. It is effectively a second line of subject copy, and left unset it shows whatever the message happens to open with.
How does an inbox choose the preheader?
By reading forward through the message until it finds text to show. That means an unset preheader displays the greeting, or worse, an image alt attribute or a view-in-browser line, because those are the first readable strings in many templates.
Marketing email controls this with a hidden preheader block at the top of the body, styled so it is invisible in the rendered message but still first in the source. How much of it appears varies by client and by device, so the useful words go at the start.
What should preheader text say in cold outreach?
Something the subject line does not. Repeating the subject wastes the only other piece of copy the recipient sees before deciding, while a preheader that extends the subject gives two short reasons to open instead of one.
Most cold email does not use a hidden preheader at all, because first-touch messages are sent as plain text to look like ordinary correspondence. In that case the first line of the body is the preheader, which is a reason to open with something substantive rather than a greeting.
Frequently asked questions
Long enough to be useful and front-loaded enough to survive truncation. Clients differ in how much they display, and the same message shows a different amount on a phone than in a desktop client, so the sentence has to make sense from its first few words.