diff --git a/internal/mail/resolve_thread_test.go b/internal/mail/resolve_thread_test.go
new file mode 100644
index 0000000..9e52251
--- /dev/null
+++ b/internal/mail/resolve_thread_test.go
@@ -0,0 +1,86 @@
+package mail
+
+import (
+	"errors"
+	"testing"
+
+	"thuanle.me/claw-email-bridge/internal/database"
+)
+
+func TestResolveThreadID_NoHeaders_ReturnsMessageID(t *testing.T) {
+	got := resolveThreadID("msg-1", "", "", nilLookup)
+	if got != "msg-1" {
+		t.Fatalf("expected msg-1, got %s", got)
+	}
+}
+
+func TestResolveThreadID_InReplyToMatches_ReturnsParentThreadID(t *testing.T) {
+	lookup := func(messageID string) (*database.Task, error) {
+		if messageID == "<parent>" {
+			return &database.Task{ThreadID: "thread-abc"}, nil
+		}
+		return nil, nil
+	}
+	got := resolveThreadID("msg-2", "<parent>", "", lookup)
+	if got != "thread-abc" {
+		t.Fatalf("expected thread-abc, got %s", got)
+	}
+}
+
+func TestResolveThreadID_ReferencesOnly_LastIDMatches(t *testing.T) {
+	lookup := func(messageID string) (*database.Task, error) {
+		if messageID == "<C>" {
+			return &database.Task{ThreadID: "thread-c"}, nil
+		}
+		return nil, nil
+	}
+	got := resolveThreadID("msg-3", "", "<A> <B> <C>", lookup)
+	if got != "thread-c" {
+		t.Fatalf("expected thread-c, got %s", got)
+	}
+}
+
+func TestResolveThreadID_ReferencesOnly_MiddleIDMatches(t *testing.T) {
+	lookup := func(messageID string) (*database.Task, error) {
+		if messageID == "<B>" {
+			return &database.Task{ThreadID: "thread-b"}, nil
+		}
+		return nil, nil
+	}
+	got := resolveThreadID("msg-4", "", "<A> <B> <C>", lookup)
+	if got != "thread-b" {
+		t.Fatalf("expected thread-b, got %s", got)
+	}
+}
+
+func TestResolveThreadID_ReferencesOnly_NoMatch_ReturnsMessageID(t *testing.T) {
+	got := resolveThreadID("msg-5", "", "<X> <Y>", nilLookup)
+	if got != "msg-5" {
+		t.Fatalf("expected msg-5, got %s", got)
+	}
+}
+
+func TestResolveThreadID_InReplyToMiss_ReferencesMatches(t *testing.T) {
+	lookup := func(messageID string) (*database.Task, error) {
+		if messageID == "<ref>" {
+			return &database.Task{ThreadID: "thread-ref"}, nil
+		}
+		return nil, nil
+	}
+	got := resolveThreadID("msg-6", "<unknown>", "<ref>", lookup)
+	if got != "thread-ref" {
+		t.Fatalf("expected thread-ref, got %s", got)
+	}
+}
+
+func TestResolveThreadID_LookupError_ReturnsMessageID(t *testing.T) {
+	lookup := func(messageID string) (*database.Task, error) {
+		return nil, errors.New("db error")
+	}
+	got := resolveThreadID("msg-7", "<parent>", "", lookup)
+	if got != "msg-7" {
+		t.Fatalf("expected msg-7, got %s", got)
+	}
+}
+
+var nilLookup = func(string) (*database.Task, error) { return nil, nil }
\ No newline at end of file