Reading Email Headers

What is an email header? What is the difference between envelope and header? Why would I need to look at email headers? How do I view full email headers in different clients (Gmail, Outlook, etc.)? How to read and interpret raw headers? What key information can I find in email headers (e.g., Received, Authentication-Results, Message-ID)? What do “Received” headers mean? What is a “Return-Path”? What is “X-Mailer” or “User-Agent”? What are custom X-headers and why use them? How can headers help troubleshoot deliverability problems? What tools can help analyze email headers? What is ARC-Seal, ARC-Message-Signature, and ARC-Authentication-Results? What is “Authentication-Results” and how to read it? What is the difference between SPF and DKIM results in headers? How to detect spoofing from headers? What is “Received-SPF”? What are “List-Unsubscribe” headers? What is “Feedback-ID”?