Will ISPs respect custom headers added to outbound emails for tracking replies when the subject line changes?

Key findings

• Custom headers: Custom headers added to outbound emails are typically not preserved or returned by mail clients in replies.
• Mail client behavior: The decision to include headers in replies rests with the email client (MUA), not the ISP.
• Subject line changes: A change in the subject line can often break email threading, especially in clients that rely heavily on the subject for conversation grouping (e.g., Gmail, in some cases).
• Standard headers: Reliable email threading depends on standard headers such as Message-ID, In-Reply-To, and References, which are designed for this purpose, as detailed in email specifications like RFC 2822.
• Alternative tracking: For tracking replies, a more reliable method involves using a unique email address in the Reply-To field, often embedding a tracking identifier or cookie within it.

Key considerations

• Reliability: Do not rely on custom headers for critical reply tracking or email threading logic.
• Standard compliance: Focus on correctly implementing and leveraging standard headers for email conversation threading. Understanding what RFC 5322 says versus what actually works can provide valuable insight.
• Reply tracking: For reply tracking, consider using unique Reply-To addresses as a more robust solution, as this approach is generally well-supported across email clients.
• Client variability: Be aware that different email clients may implement threading logic differently, leading to varied behavior, particularly when subject lines change. You may also be interested in how email replies and 'no-reply' addresses affect deliverability.

Key opinions

• Custom headers are unreliable: Many marketers have found that custom headers in outbound emails are not consistently returned in replies by mail clients, making them unsuitable for reliable tracking.
• Subject line impact: It's a common observation that if an email's subject line changes in a reply, some email clients, like Gmail, may treat it as a new conversation rather than continuing the original thread.
• Client-specific threading: Marketers acknowledge that different email clients employ varied algorithms for email threading, with some using standard In-Reply-To headers and others relying more heavily on subject lines.
• Reply-to address as a solution: A practical suggestion for tracking replies is to embed a unique identifier or cookie within the Reply-To email address itself.
• Focus on standard headers: While custom headers allow for various functionalities like customizing or tagging messages, their utility for tracking replies is limited compared to standard headers.

Key considerations

• Avoid over-reliance: Marketers should avoid relying heavily on custom headers for tracking email replies, as this method is prone to failure across different email clients.
• Embrace standard practices: For robust email threading, adhere to email standards that utilize Message-ID, In-Reply-To, and References.
• Implement Reply-To tracking: For accurate reply tracking, dynamically generate a unique Reply-To address for each recipient or email, as this provides a direct and reliable tracking mechanism. This can also impact deliverability and sender reputation.
• Understand ISP tracking: While ISPs monitor engagement, their methods for tracking email engagement are distinct from how mail clients handle reply headers.

Key opinions

• Client-side decision: Experts affirm that whether a custom header is returned in a reply is a decision made by the mail client, not the ISP. ISPs merely facilitate the transmission of emails.
• Headers not returned: Custom headers are typically not included in replies because a reply essentially forms a new MIME structure with its own set of headers generated by the MUA.
• Threading complexity: Email clients thread messages differently. While some rely on subject lines (like Gmail, which can cause threads to break if the subject changes), others use more robust headers such as In-Reply-To and References to maintain conversation context.
• Reliable tracking alternative: Using an email address in the Reply-To field that includes a unique identifier or "cookie" is considered a more reliable method for tracking specific replies.
• Adherence to standards: Experts recommend consulting IETF specifications for MUAs and headers to understand the intended behavior, even if full implementation is not universal. Understanding how to read email headers is also beneficial.

Key considerations

• Discard custom header tracking: Experts advise against attempting to track email replies through custom headers, as this approach is fundamentally flawed due to MUA behavior.
• Leverage Reply-To: Implement unique identifiers within the Reply-To field to reliably associate incoming replies with specific outbound messages.
• Header importance: While custom headers may not persist, understanding the structure and impact of other headers, such as header.i in DKIM, remains crucial for sender reputation.
• Personalization errors: Pay attention to how personalization errors in email headers can affect deliverability, even if they aren't directly related to reply tracking.

Key findings

• RFC 2822: This foundational document, also known as the Internet Message Format, outlines the standard headers required for email messages, including those for threading.
• Message-ID: Every email should have a unique Message-ID header, which serves as its primary identifier in a conversation.
• In-Reply-To and References: These headers explicitly link a reply to one or more previous messages in a conversation, ensuring proper threading even if the subject changes. This is a core mechanism for email systems.
• Custom headers: While RFCs allow for X-headers (custom headers), they do not mandate their preservation or return in replies by mail clients, indicating they should not be relied upon for critical message continuity.
• MUA responsibility: Mail User Agents (email clients) are responsible for correctly generating these standard threading headers in replies to ensure messages are grouped properly in conversation views.

Key considerations

• Adherence to standards: For reliable email communication and threading, it is crucial to adhere to existing email standards and RFCs. This forms the basis of all email interactions.
• Limited custom header utility: Custom headers are useful for specific application-level data but should not be used for functionality that relies on them being returned in replies.
• Comprehensive understanding: A thorough understanding of how DMARC, SPF, and DKIM work, alongside message headers, is essential for overall email deliverability. For a deeper dive, consider email explained from first principles.
• Technical solutions: When seeking to track replies, consider technical solutions that are compliant with RFCs and widely adopted practices, rather than relying on non-standard header propagation. These insights are part of technical solutions from top performing senders.