Which field in an email header tells you the name of the sending domain from which the message was sent?

When you view the "original" or "headers" of an email (especially in Gmail), the mailed-by field is the primary indicator of the technical domain used to transmit the message. The Sending Domain: This field displays the domain found in the SPF (Sender Policy Framework) record. It tells the recipient's inbox provider exactly which server was responsible for physically moving the email. The "via" Tag: If you are using a shared domain (like klaviyomail.com), Gmail will often show your name followed by "via klaviyomail.com." This happens because the mailed-by field does not match your brand's domain. Branded Domains: Once you set up a branded sending domain, the mailed-by field will change to match your own subdomain (e.g., send.yourbrand.com). This removes the "via" text and makes your emails look more professional and trustworthy to spam filters.