Email threading headers (In-Reply-To, References, Message-ID)
Most modern ticket systems parse these headers to maintain conversation threads regardless of subject line changes. When someone replies to an email, their client includes these headers pointing back to the original message. If that original message belongs to ticket #123, the reply should too.
This would handle:
Users who reply without preserving ticket references
Forward chains where subject gets mangled
Mobile clients that strip/modify subjects
Why subject-only matching is risky:
Multiple users reporting "server down" would merge incorrectly
Generic subjects like "question" or "issue" create false positives
Legitimate separate issues from same user get combined
Possible hybrid approach:
Primary: Match via threading headers (most reliable)
Fallback: Match via ticket reference in subject (current behavior)
Optional: Time-window + sender + subject fuzzy match with configurable threshold