Free Email Header Validator (RFC 5322 Compliance)
Validate email header hygiene and RFC 5322 compliance. Check Return-Path alignment, Message-ID format, and bulk email headers.
Email Header Validator
Validate email header hygiene and RFC 5322 compliance
What is Email Header Validation?
Overview
Email headers contain critical metadata that affects deliverability, authentication, and compliance. Proper header hygiene ensures your emails comply with RFC 5322 standards, align with DMARC authentication, and include required fields for bulk email sending. Poor header configuration can trigger spam filters, cause authentication failures, or violate anti-spam regulations.
Critical Email Headers
Return-Path
Specifies where bounce messages should be sent. Must align with the From domain for DMARC authentication. Misalignment is a common deliverability issue.
Message-ID
Unique identifier for each email. Required by RFC 5322. Format must be <localpart@domain> with proper angle brackets.
Date
Timestamp when the email was sent. Must be in RFC 5322 format. Date more than 24 hours in the future or past can trigger spam filters.
List-Unsubscribe
Required for bulk email. Provides unsubscribe mechanism. Must include List-Unsubscribe-Post header for one-click unsubscribe compliance.
Return-Path Alignment
For DMARC to pass, the Return-Path (envelope from) domain must align with the From header domain. There are two types of alignment:
Strict Alignment
✓ Domains must match exactly: bounce@example.com and sender@example.com
Relaxed Alignment
✓ Organizational domains must match: bounce@mail.example.com and sender@example.com
Bulk Email Headers (Required)
If you send marketing, newsletters, or any commercial email, you must include these headers to comply with anti-spam laws (CAN-SPAM, CASL, GDPR):
- List-Unsubscribe: URL or mailto link for unsubscribing
- List-Unsubscribe-Post: Enables one-click unsubscribe (required by Gmail/Yahoo 2024)
- List-ID: Identifies the mailing list
- Precedence: bulk: Marks the email as bulk/marketing
Common Header Issues
Missing Message-ID
Every email must have a unique Message-ID. Missing or duplicate IDs trigger spam filters and violate RFC 5322.
Return-Path Misalignment
Return-Path domain doesn't match From domain, causing DMARC failures. Use the same domain or proper subdomain alignment.
Invalid Date Format
Date header not in RFC 5322 format or significantly different from current time. Can indicate spam or misconfigured server.
Missing List-Unsubscribe
Bulk emails without unsubscribe headers violate anti-spam laws and Gmail/Yahoo requirements. Must include both List-Unsubscribe and List-Unsubscribe-Post.
One-Click Unsubscribe (RFC 8058)
As of 2024, Gmail and Yahoo require one-click unsubscribe for bulk senders. This requires two headers:
Header Best Practices
- Always include a unique Message-ID with your sending domain
- Ensure Return-Path aligns with From domain for DMARC compliance
- Use accurate Date headers synchronized with actual send time
- Include List-Unsubscribe and List-Unsubscribe-Post for bulk email
- Add Precedence: bulk header for newsletters and marketing
- Include List-ID to identify your mailing list
- Avoid special characters or encoding issues in headers
- Keep header line lengths under 998 characters (RFC 5322)
Impact on Deliverability
- DMARC Authentication: Return-Path misalignment causes DMARC failures
- Spam Filters: Missing or malformed headers trigger content filters
- Compliance: Missing List-Unsubscribe violates CAN-SPAM and GDPR
- Reputation: Poor header hygiene signals unprofessional or spammy sender
- User Experience: One-click unsubscribe improves recipient satisfaction