How to structure headers for transactional compliance?

Proper header structure for transactional emails emphasizes clarity and deliverability while ensuring compliance with applicable requirements. The "From" header should clearly identify your organization-recipients should recognize the sender, and the address should be monitored for replies. The "Reply-To" header should route to a functional address where someone will actually see and respond to messages, particularly for service-oriented transactional emails where recipients might have questions about their order or account.

Transactional emails should include List-Unsubscribe headers for marketing content, even when the message is primarily transactional. If your transactional email includes promotional elements, implementing List-Unsubscribe allows recipients to opt out of the promotional portion while continuing to receive necessary transactional communications. This respects recipient preferences and signals to mailbox providers that you're operating responsibly. For purely transactional emails without promotional content, List-Unsubscribe isn't required but may still be appropriate for messages like digests or alerts that recipients might want to adjust.

Additional header considerations include Precedence headers (some senders use "Precedence: transactional" or "Precedence: bulk" to help receivers categorize messages), X-headers for internal tracking and categorization (X-Campaign-Type: transactional), and proper DKIM, SPF, and DMARC authentication to ensure deliverability. Transactional emails often warrant separate DKIM selectors or sending domains to isolate their reputation from marketing sends. Headers tell mailbox providers what kind of message you're sending-structure them honestly and technically correctly to ensure your transactional emails reach their intended recipients.