What key information can I find in email headers (e.g., Received, Authentication-Results, Message-ID)?

Headers contain a wealth of information. The most useful fields for deliverability:

Received: Records each server hop. Shows IP addresses, hostnames, timestamps, and protocols used. Essential for tracing routing problems.

Authentication-Results: The receiving server's verdict on SPF, DKIM, and DMARC. Shows pass/fail/none for each and often includes the specific domain or selector checked.

Message-ID: A globally unique identifier assigned by the originating server. Format: <unique-string@domain>. Useful for tracking messages through logs.

Return-Path: The envelope sender, where bounces go. May differ from the From header.

DKIM-Signature: The actual DKIM signature with selector, domain, and signed header list. Receiving servers use this to verify authenticity.

X-Spam-Status / X-Spam-Score: Many servers add spam filtering results. Shows the score and which rules triggered.

X-Originating-IP: Some providers record the original sender's IP (useful for webmail where the user's IP differs from the server).

Feedback-ID: Used by ESPs to identify campaigns and senders within feedback loop reports.

Think of it as reading a detailed travel itinerary. Every stamp, checkpoint, and inspection is recorded.