Repair Email Threading, Replies & Conversation Mapping

The likely issue is a contract mismatch around email Message-ID/In-Reply-To/References handling, provider threading, aliases, plus-addressing, forwarded mail, duplicate delivery, subject changes, conversation ownership, and reply routing. I would first define the invariant: Conversation membership must derive from authenticated tenant/contact context plus durable message/thread references; subjects are presentation metadata and may only be a conservative fallback when stronger identifiers are unavailable. Then I would trace the smallest owning boundary, apply or propose a bounded repair/review step, and add deterministic normal, boundary, failure and replay tests. The workflow preserves tenant/customer isolation, reply-to policy, privacy, existing provider contracts, and user-visible conversation history. If repository/workspace files are available, the findings can be mapped to exact paths and focused local validation; with partial evidence, the result stays an explicit diagnosis/evidence map with assumptions and no invented execution claims.