diff --git a/internal/httpapi/handlers_projects.go b/internal/httpapi/handlers_projects.go
index 5e860fe..ea428af 100644
--- a/internal/httpapi/handlers_projects.go
+++ b/internal/httpapi/handlers_projects.go
@@ -603,6 +603,9 @@ func (s *Server) handleListProjectMails(w http.ResponseWriter, r *http.Request)
 	if st.LastError != "" {
 		resp["error"] = st.LastError
 	}
+	if isSyncing(id) {
+		resp["syncing"] = true
+	}
 	// First ever view → kick off full backfill in the background (only if mail
 	// integration is configured). Already-stored mail is shown regardless.
 	if !synced && s.mail.Enabled() {
@@ -619,10 +622,24 @@ func (s *Server) handleListProjectMails(w http.ResponseWriter, r *http.Request)
 		notes[n.MessageID] = n
 	}
 	me := s.email(r)
+	// 가시성: 내가 참여한 "스레드 전체"를 노출(원문 + 답장 각각). 한 스레드에서
+	// 한 통이라도 내가 from/to/cc면 그 스레드의 모든 메일이 보인다.
+	myThreads := map[string]bool{}
+	involvedSolo := map[string]bool{}
+	for _, row := range rows {
+		m := mailsync.Message{From: row.FromAddr, To: row.ToAddr, Cc: row.CcAddr}
+		if m.Involves(me) {
+			if row.ThreadID != "" {
+				myThreads[row.ThreadID] = true
+			} else {
+				involvedSolo[row.MessageID] = true
+			}
+		}
+	}
 	items := make([]mailItem, 0, len(rows))
 	for _, row := range rows {
-		m := mailsync.Message{ID: row.MessageID, From: row.FromAddr, To: row.ToAddr, Cc: row.CcAddr}
-		if !m.Involves(me) {
+		visible := involvedSolo[row.MessageID] || (row.ThreadID != "" && myThreads[row.ThreadID])
+		if !visible {
 			continue
 		}
 		it := mailItem{ProjectMailMsg: row}
diff --git a/internal/httpapi/mail_sync.go b/internal/httpapi/mail_sync.go
index 44f45d4..8d2e3f9 100644
--- a/internal/httpapi/mail_sync.go
+++ b/internal/httpapi/mail_sync.go
@@ -16,6 +16,12 @@ import (
 // syncInFlight prevents concurrent syncs of the same project.
 var syncInFlight sync.Map // projectID -> struct{}
 
+// isSyncing reports whether a sync is currently running for the project.
+func isSyncing(projectID string) bool {
+	_, ok := syncInFlight.Load(projectID)
+	return ok
+}
+
 // syncProjectMail pulls the project's client-domain mail across ALL active member
 // mailboxes (domain-wide delegation), upserts the headers into ProjectMailMsg, and
 // records sync state. full=true pages the entire history; otherwise just the newest