Understanding headers is key to diagnosing deliverability issues.
| Header | Purpose |
|---|---|
| From | Visible sender address; used in DMARC alignment |
| Return-Path | Envelope sender; used for SPF alignment and bounces |
| Reply-To | Where replies go (can differ from From) |
| Message-ID | Unique identifier for threading |
| Received | Trace route — read bottom-to-top |
| Authentication-Results | SPF, DKIM, DMARC pass/fail results |
| DKIM-Signature | Cryptographic signature details |
| List-Unsubscribe | Unsubscribe URIs (required for bulk senders) |
⚠️ Key insight: The Return-Path (used for SPF) and the From header (used for DMARC alignment) are often different domains. This is the #1 source of authentication confusion.
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