Understanding Email Headers for Troubleshooting

What are Email Headers?

Email headers contain metadata about a message — who sent it, which servers handled it, authentication results, and timestamps. Reading headers is essential for diagnosing delivery issues.

How to View Headers

• Gmail — open message → three dots → "Show original"
• Outlook — open message → File → Properties → "Internet headers"
• Command line — cat message.eml | head -50

Key Headers

From: sender@example.com
To: recipient@example.com
Date: Tue, 25 Feb 2026 10:30:00 -0500
Subject: Test Email
Message-ID: <unique-id@example.com>
Return-Path: <sender@example.com>

Authentication Headers

# SPF result
Received-SPF: pass (google.com: domain of sender@example.com designates 198.51.100.10 as permitted sender)

# DKIM result
DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=mail; ...
Authentication-Results: mx.google.com; dkim=pass; spf=pass; dmarc=pass

Received Headers

Read bottom-to-top — they trace the email path from sender to recipient:

Received: from mail.example.com (198.51.100.10) by mx.google.com
  with ESMTPS; Tue, 25 Feb 2026 10:30:05 -0500

Troubleshooting

• Delays — compare timestamps in Received headers to find slow hops
• SPF failures — check if the sending IP matches your SPF record
• DKIM failures — verify your DKIM selector and public key