What does the term “email header” mean in the context of deliver ability?

n the technical world of deliverability, an email header (also known as an internet header) is the "metadata" or "passport" that accompanies every email. While it is mostly hidden from the average reader, it is the first thing an inbox provider looks at to decide if your message is safe. Technical Blueprint: The header is a block of code at the very top of the email's source. It records every server the email passed through on its way from your ESP (like Klaviyo) to the recipient's inbox. Authentication Results: This is where the results of your SPF, DKIM, and DMARC checks live. If an inbox provider sees a "Pass" in the header for these authentication protocols, they are much more likely to deliver the email to the inbox. Spam Scoring: Inbox providers like Gmail and Yahoo use the header to assign a "Spam Score." If your header contains suspicious information—like a mismatched "From" address and "Reply-To" address—this score increases, and your email is more likely to be sent to spam.