Summary
Experts, marketers, and documentation sources overwhelmingly agree that relying on custom headers for tracking email replies when the subject line changes is unreliable. ISPs and email clients frequently strip or modify these headers for security, standardization, and interoperability reasons. Email threading primarily depends on standard headers like 'In-Reply-To', 'References', and the 'Subject' header, with variations in how different clients handle threading. Custom headers might also be ignored by spam filters or even used as spam signals. Alternative tracking methods and a focus on standard headers are recommended.
Key findings
- Unreliable Custom Headers: Custom headers are generally unreliable for tracking replies due to frequent stripping or modification by ISPs and email clients.
- Standard Header Reliance: Email threading primarily relies on standard headers like 'In-Reply-To', 'References', and the 'Subject' header.
- Client Variations: Email clients exhibit variations in how they handle email threading and header interpretation.
- Spam Filter Concerns: Custom headers may be ignored by spam filters or treated as spam signals.
- Subject Line Impact: Changing the subject line often breaks email threads, rendering custom header tracking ineffective.
Key considerations
- Use Standard Headers: Prioritize the use of standard headers for reliable tracking.
- Alternative Tracking: Explore alternative tracking methods that don't depend on custom headers, like cookies or Message-ID references.
- Preserve Subject Line: Avoid changing the subject line of emails in a thread to maintain continuity.
- Account for Stripping: Recognize that custom headers may be stripped by ISPs and email clients, rendering them ineffective.
- Spam Filter Awareness: Consider the potential impact of custom headers on spam filter behavior.
What email marketers say
9 marketer opinions
The consensus is that relying on custom headers for tracking email replies when the subject line changes is unreliable. ISPs and email clients often strip or modify these headers, and thread tracking primarily depends on standard headers like 'In-Reply-To' and 'References'. Changing the subject line can break email threads, and custom headers offer little to no help in preventing this.
Key opinions
- Header Stripping: ISPs and email clients frequently strip or modify custom headers for security, standardization, or other reasons.
- Thread Dependency: Email threading primarily relies on 'In-Reply-To', 'References', and the 'Subject' header. Changing the subject line can break the thread.
- Limited Support: Custom headers have inconsistent support across different email clients and servers.
- Not Guaranteed: Custom headers are not guaranteed to be preserved or respected.
Key considerations
- Standard Headers: Focus on using standard email headers for reliable thread tracking.
- Subject Line: Avoid changing the subject line in email threads to maintain continuity.
- Alternative Tracking: Explore alternative tracking methods that do not depend on custom headers.
- Client Variations: Be aware that different email clients handle headers differently.
Marketer view
Email marketer from Email on Acid explains that different email clients and ISPs have varying levels of support for custom headers. Some may strip them out entirely, while others may modify them. This inconsistency makes custom headers unreliable for tracking purposes.
11 Mar 2025 - Email on Acid
Marketer view
Email marketer from Campaign Monitor shares that relying on custom headers for tracking is risky due to inconsistent support across email clients and servers. Best practices include using standard tracking methods and carefully managing subject lines.
20 Nov 2024 - Campaign Monitor
What the experts say
4 expert opinions
Experts across various platforms agree that relying on custom headers for tracking email replies when the subject line changes is not a reliable strategy. ISPs and email clients often strip or modify these headers. Email clients thread differently, and some rely on subject lines or 'In-Reference-To' headers. While custom headers can be used, they might be ignored by spam filters or treated as spam signals. Using standard headers is generally more reliable, but even these can be subject to change. Alternative methods like cookies in the Reply-To field or original quoted mail might be more effective.
Key opinions
- Custom Header Unreliability: Custom headers are generally unreliable for tracking replies due to stripping or modification by ISPs and email clients.
- Varying Threading Methods: Email clients use different methods for threading, some relying on subject lines or 'In-Reference-To' headers.
- Spam Filter Impact: Custom headers may be ignored by spam filters or trigger spam signals.
- Standard Header Preference: Standard headers are more likely to be respected, but even they can be subject to change.
Key considerations
- Alternative Tracking Methods: Consider using alternative tracking methods like cookies in the Reply-To field or original quoted mail.
- Standard Header Usage: Prioritize the use of standard headers over custom headers.
- Awareness of Threading Behavior: Understand how different email clients handle email threading.
- Spam Filter Implications: Be mindful of how custom headers might affect spam filter behavior.
Expert view
Expert from Word to the Wise explains that custom headers are not reliable for tracking replies, as they are often stripped or modified by ISPs or email clients. Standard headers are more likely to be respected, but even they can be subject to change.
16 Aug 2024 - Word to the Wise
Expert view
Expert from Spam Resource shares that while custom headers can be used, they are often ignored by spam filters or are used as signals for potential spam. They are not a reliable method for ensuring delivery or tracking replies, especially if the subject line changes.
23 Aug 2024 - Spam Resource
What the documentation says
5 technical articles
Documentation from various sources indicates that ISPs and email clients are not guaranteed to respect custom headers added to outbound emails, especially if the subject line changes. Standard headers are more consistently handled, but custom headers can be modified or removed for security, compliance, or interoperability reasons. Gmail's threading relies primarily on the Subject and References headers, and if those are disrupted, a new thread may start. Reliable tracking should focus on standard headers and parameters rather than relying on custom headers.
Key findings
- Custom Header Uncertainty: Custom headers are not guaranteed to be preserved or interpreted consistently across different email systems.
- Potential Modification: Exchange Online and other systems may modify or remove custom headers for security and compliance purposes.
- Threading Dependence: Gmail's threading algorithm relies heavily on Subject and References headers; changes can break threads.
- Reliability Concerns: There is no assurance that receiving servers or clients will preserve custom headers.
Key considerations
- Standard Headers First: Prioritize standard headers and parameters for tracking emails, as they are more reliably supported.
- Subject Line Impact: Be aware that changing the subject line can negatively impact email threading and tracking.
- Compliance Factors: Consider that email systems may alter or remove custom headers to meet security or compliance requirements.
- System Differences: Recognize that the treatment of custom headers can vary across different email clients and servers.
Technical article
Documentation from Microsoft shares that Exchange Online may modify or remove custom headers for security or compliance reasons, particularly in outbound emails. This behavior can affect the reliability of custom headers for tracking replies.
30 Aug 2023 - Microsoft
Technical article
Documentation from Google Groups explains that Gmail's threading algorithm primarily relies on the 'Subject' and 'References' headers. If the subject line changes drastically, or the 'References' header is missing or incorrect, Gmail may start a new thread.
20 Dec 2022 - Google Groups
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