Improve email threading when ticket subject changes

Currently, when an agent updates a ticket's subject in Zendesk, subsequent email notifications to the customer often create a separate email thread. This happens because many email clients rely on the subject line for threading instead of the unique reply-to address (e.g., support+idVWDDDG-42XXX@COMPANY.zendesk.com).

Issue:

• Despite the unique reply-to address ensuring responses route to the correct ticket, email clients frequently break the thread when the subject changes.
• This leads to fragmented conversations, making it harder for customers and agents to track discussions.

Workaround & Limitations:

• A workaround exists by modifying the notification trigger to replace {{ ticket.title }} in the subject with a static subject (e.g., "Your support ticket update") and leaving a {{ ticket.id }}for the reference.
• However, this workaround removes the subject context from notification emails, which may be important for customers.

Improvement:
Zendesk should enhance its email threading logic to prioritize the unique reply-to address over the subject line for matching replies to the same thread. This would prevent email clients from mistakenly splitting conversations when an agent updates the ticket title.

Benefits:

• Ensures continuity in email threads, improving customer experience.
• Reduces confusion and the risk of missing important context in separate email chains.
• Aligns with best practices for email-based ticketing systems.

Would it be possible for Zendesk to introduce this improvement, or is there a technical limitation preventing such a change?