Remembers the last inbound message per email conversation so an outbound reply can thread against it.

Raxol.Gateway.Adapter.Email.send_message/3 cannot learn the inbound Message-ID from its arguments -- the Raxol.Gateway.Adapter contract is frozen at (conn, route, rendered). This store closes that gap: the inbound feed's wiring records each normalized message here (keyed by the conversation address), and the adapter reads it back at send time through the conn's :thread_lookup function, which thread_lookup_fn/1 builds.

The store is a small in-memory cap map (one entry per conversation address, most-recently-seen wins), bounded by :max_entries (default 10_000) with oldest-recorded eviction. It holds only mail metadata (Message-ID, references, subject) -- never message bodies.

Wiring

{:ok, store} = ThreadStore.start_link(name: MyThreads)

on_message = fn raw ->
  case Email.normalize_event(raw) do
    {:ok, route, event} ->
      ThreadStore.record_event(store, route, event)
      SessionRouter.route(router, route, event)

    :ignore ->
      :ok
  end
end

{:ok, conn} =
  Email.connect(
    relay: relay,
    from: from,
    thread_lookup: ThreadStore.thread_lookup_fn(store)
  )