An email header is the set of metadata fields attached to every email message that contains technical information about how the message was created, routed, and delivered. Headers include the sender and recipient addresses, subject line, date, message ID, and detailed routing information showing each server the email passed through. They also contain authentication records like SPF, DKIM, and DMARC results that verify the sender's identity.
Email headers serve important functions for both deliverability and security. Email service providers examine headers to authenticate the sender, detect spoofing attempts, and make filtering decisions. When an email fails authentication checks recorded in the header, it is more likely to be flagged as spam or rejected. For sales teams sending cold outreach, properly configured headers are essential they prove your emails are legitimate and improve the chances of reaching the inbox.
Understanding email headers is also valuable for troubleshooting deliverability issues. When emails are not being received, examining the headers of test messages can reveal where in the delivery chain problems occur.
Key Points
- Email headers contain routing, authentication, and metadata for every email message
- Authentication records in headers (SPF, DKIM, DMARC) verify sender identity and improve deliverability
- Examining headers helps diagnose delivery issues and confirm proper email configuration
Best Practices
- Configure SPF, DKIM, and DMARC records properly to ensure your email headers pass authentication
- Review email headers when troubleshooting deliverability problems to identify the failure point
- Ensure your sending infrastructure produces clean, well-formed headers that pass provider checks