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

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.

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.

Email marketer from Litmus explains that if the subject line of an email is changed, it can lead to "thread hijacking," where replies are no longer associated with the original email. Custom headers are unlikely to prevent this.

Email marketer from NeilPatel.com shares that while custom headers can be used for internal tracking, ISPs and email clients may strip them out or not pass them along in replies, especially if the subject line changes. Relying on them for thread tracking is not a reliable strategy.

Email marketer from SuperOffice explains that changing the subject line of an email can break the email thread, and while some clients use other headers to maintain the thread, custom headers are not typically considered for this purpose and may be stripped.

Email marketer from Mailjet explains that ISPs and email clients often use the 'In-Reply-To' and 'References' headers to manage email threads. Custom headers are less likely to be respected, especially if the subject line is modified, which can break the thread.

Email marketer from Reddit shares that using custom headers for tracking replies is generally unreliable. Modern email clients and servers often strip or modify headers, making it difficult to maintain consistent tracking across different systems.

Email marketer from Stack Overflow explains that custom headers are not guaranteed to be preserved by all email clients and servers. Some may strip them out for various reasons, including security and standardization. Reliance on custom headers for crucial functionality is discouraged.

Email marketer from Email Geeks shares that Gmail uses both subject and in `In-Reply-To` headers (and `References`) — if they exist. When the subject changes, Any random or other headers will not survive a reply as replies are in essence a brand new MIME structure with its own headers, usually created by the MUA.

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.

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.

Expert from Email Geeks explains that mail clients don’t return all the headers back in the reply and different clients thread differently - some use In-Reference-To: headers which survive a subject line change, and some use subject lines (like it appears Gmail is). Adding a custom header probably won't achieve the desired tracking outcome.

Expert from Email Geeks explains that ISPs will not include a custom header in replies. Most of the time, one should get some reference to the original Message-ID. An email address in the Reply-To field that includes a cookie or stashing cookies in the original quoted mail may be the best bet.

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.

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.

Documentation from IETF specifies that email systems might modify or remove header fields, especially custom ones, to ensure interoperability or prevent security issues. The survival of custom headers is not guaranteed.

Documentation from SendGrid shares that while custom headers can be added to emails, there's no guarantee that they will be preserved by receiving email servers or clients. For reliable tracking, it's best to use standard headers and parameters.

Documentation from ietf.org explains that standard headers defined in RFC2822 are expected to be handled consistently by mail user agents (MUAs), but custom headers are not guaranteed to be preserved or interpreted in a predictable manner across different systems.