diff --git a/internal/store/seenset.go b/internal/store/seenset.go
new file mode 100644
index 0000000..bc4d5c9
--- /dev/null
+++ b/internal/store/seenset.go
@@ -0,0 +1,154 @@
+package store
+
+import "database/sql"
+
+// EnsureFolderBaseline initialises folder_state on first contact, or resets it
+// when the server's UIDVALIDITY differs from what we stored.
+func (s *Store) EnsureFolderBaseline(account, folder string, uidvalidity, maxUID uint32) error {
+	a, err := s.GetAccount(account)
+	if err != nil {
+		return err
+	}
+	var (
+		curUIDValidity uint32
+		haveRow        bool
+	)
+	row := s.db.QueryRow(
+		"SELECT uidvalidity FROM folder_state WHERE account_id=? AND folder=?", a.ID, folder)
+	switch err := row.Scan(&curUIDValidity); err {
+	case nil:
+		haveRow = true
+	case sql.ErrNoRows:
+		haveRow = false
+	default:
+		return err
+	}
+	if haveRow && curUIDValidity == uidvalidity {
+		return nil // already baselined, same validity
+	}
+
+	floor := maxUID
+	if a.ProcessBacklog {
+		floor = 0
+	}
+	tx, err := s.db.Begin()
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback()
+	if _, err := tx.Exec("DELETE FROM acked WHERE account_id=? AND folder=?", a.ID, folder); err != nil {
+		return err
+	}
+	if _, err := tx.Exec(`
+		INSERT INTO folder_state(account_id,folder,uidvalidity,floor_uid)
+		VALUES(?,?,?,?)
+		ON CONFLICT(account_id,folder) DO UPDATE SET uidvalidity=excluded.uidvalidity, floor_uid=excluded.floor_uid`,
+		a.ID, folder, uidvalidity, floor); err != nil {
+		return err
+	}
+	return tx.Commit()
+}
+
+func (s *Store) floor(account, folder string) (uint32, error) {
+	id, err := s.accountID(account)
+	if err != nil {
+		return 0, err
+	}
+	var f uint32
+	err = s.db.QueryRow(
+		"SELECT floor_uid FROM folder_state WHERE account_id=? AND folder=?", id, folder).Scan(&f)
+	return f, err
+}
+
+// IsNew reports whether uid is unread: above the floor and not acked.
+func (s *Store) IsNew(account, folder string, uid uint32) (bool, error) {
+	id, err := s.accountID(account)
+	if err != nil {
+		return false, err
+	}
+	var floor uint32
+	if err := s.db.QueryRow(
+		"SELECT floor_uid FROM folder_state WHERE account_id=? AND folder=?", id, folder).Scan(&floor); err != nil {
+		return false, err
+	}
+	if uid <= floor {
+		return false, nil
+	}
+	var one int
+	err = s.db.QueryRow(
+		"SELECT 1 FROM acked WHERE account_id=? AND folder=? AND uid=?", id, folder, uid).Scan(&one)
+	if err == sql.ErrNoRows {
+		return true, nil
+	}
+	if err != nil {
+		return false, err
+	}
+	return false, nil // present in acked
+}
+
+// FilterNew returns the subset of uids that are new, preserving order.
+func (s *Store) FilterNew(account, folder string, uids []uint32) ([]uint32, error) {
+	out := make([]uint32, 0, len(uids))
+	for _, u := range uids {
+		n, err := s.IsNew(account, folder, u)
+		if err != nil {
+			return nil, err
+		}
+		if n {
+			out = append(out, u)
+		}
+	}
+	return out, nil
+}
+
+// Ack records uids as processed (ignoring any at or below the floor) then compacts.
+func (s *Store) Ack(account, folder string, uidvalidity uint32, uids ...uint32) error {
+	id, err := s.accountID(account)
+	if err != nil {
+		return err
+	}
+	tx, err := s.db.Begin()
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback()
+
+	var floor uint32
+	if err := tx.QueryRow(
+		"SELECT floor_uid FROM folder_state WHERE account_id=? AND folder=?", id, folder).Scan(&floor); err != nil {
+		return err
+	}
+	for _, u := range uids {
+		if u <= floor {
+			continue
+		}
+		if _, err := tx.Exec(
+			"INSERT OR IGNORE INTO acked(account_id,folder,uidvalidity,uid) VALUES(?,?,?,?)",
+			id, folder, uidvalidity, u); err != nil {
+			return err
+		}
+	}
+	// Compact: while floor+1 is acked, advance floor and drop that row.
+	for {
+		next := floor + 1
+		var present int
+		err := tx.QueryRow(
+			"SELECT 1 FROM acked WHERE account_id=? AND folder=? AND uid=?", id, folder, next).Scan(&present)
+		if err == sql.ErrNoRows {
+			break
+		}
+		if err != nil {
+			return err
+		}
+		if _, err := tx.Exec(
+			"DELETE FROM acked WHERE account_id=? AND folder=? AND uid=?", id, folder, next); err != nil {
+			return err
+		}
+		floor = next
+	}
+	if _, err := tx.Exec(
+		"UPDATE folder_state SET floor_uid=? WHERE account_id=? AND folder=?", floor, id, folder); err != nil {
+		return err
+	}
+	return tx.Commit()
+}
diff --git a/internal/store/seenset_test.go b/internal/store/seenset_test.go
new file mode 100644
index 0000000..7af4562
--- /dev/null
+++ b/internal/store/seenset_test.go
@@ -0,0 +1,107 @@
+package store
+
+import "testing"
+
+func backlogAccount(t *testing.T, s *Store, backlog bool) {
+	t.Helper()
+	a := sampleAccount()
+	a.ProcessBacklog = backlog
+	if _, err := s.AddAccount(a); err != nil {
+		t.Fatalf("AddAccount: %v", err)
+	}
+}
+
+func TestBaselineIgnoresHistoryByDefault(t *testing.T) {
+	s := openTemp(t)
+	backlogAccount(t, s, false)
+	if err := s.EnsureFolderBaseline("work", "INBOX", 1, 100); err != nil {
+		t.Fatalf("baseline: %v", err)
+	}
+	// Existing mail (uid <= 100) is not new; 101 is.
+	for _, tc := range []struct {
+		uid  uint32
+		want bool
+	}{{50, false}, {100, false}, {101, true}} {
+		got, _ := s.IsNew("work", "INBOX", tc.uid)
+		if got != tc.want {
+			t.Fatalf("IsNew(%d)=%v want %v", tc.uid, got, tc.want)
+		}
+	}
+}
+
+func TestBaselineBacklogProcessesAll(t *testing.T) {
+	s := openTemp(t)
+	backlogAccount(t, s, true)
+	_ = s.EnsureFolderBaseline("work", "INBOX", 1, 100)
+	if n, _ := s.IsNew("work", "INBOX", 1); !n {
+		t.Fatal("with backlog, uid 1 must be new")
+	}
+}
+
+func TestAckRemovesFromNewAndIsOutOfOrderSafe(t *testing.T) {
+	s := openTemp(t)
+	backlogAccount(t, s, false)
+	_ = s.EnsureFolderBaseline("work", "INBOX", 1, 100)
+	// New mail arrives: 101..105. Ack out of order: 103 then 101.
+	_ = s.Ack("work", "INBOX", 1, 103)
+	if n, _ := s.IsNew("work", "INBOX", 103); n {
+		t.Fatal("103 acked, must not be new")
+	}
+	if n, _ := s.IsNew("work", "INBOX", 101); !n {
+		t.Fatal("101 not acked yet, must still be new")
+	}
+	_ = s.Ack("work", "INBOX", 1, 101, 102)
+	got, _ := s.FilterNew("work", "INBOX", []uint32{101, 102, 103, 104, 105})
+	want := []uint32{104, 105}
+	if len(got) != 2 || got[0] != want[0] || got[1] != want[1] {
+		t.Fatalf("FilterNew got %v want %v", got, want)
+	}
+}
+
+func TestCompactionAdvancesFloor(t *testing.T) {
+	s := openTemp(t)
+	backlogAccount(t, s, false)
+	_ = s.EnsureFolderBaseline("work", "INBOX", 1, 100)
+	// Ack a contiguous run just above the floor: 101,102,103.
+	_ = s.Ack("work", "INBOX", 1, 102, 101, 103)
+	f, _ := s.floor("work", "INBOX")
+	if f != 103 {
+		t.Fatalf("floor should advance to 103, got %d", f)
+	}
+	// The acked rows for the collapsed run are gone.
+	var n int
+	_ = s.db.QueryRow("SELECT COUNT(*) FROM acked").Scan(&n)
+	if n != 0 {
+		t.Fatalf("acked rows should be compacted away, got %d", n)
+	}
+	// A hole remains uncompacted: ack 105 (gap at 104).
+	_ = s.Ack("work", "INBOX", 1, 105)
+	f, _ = s.floor("work", "INBOX")
+	if f != 103 {
+		t.Fatalf("floor must stay at 103 while 104 is a hole, got %d", f)
+	}
+	_ = s.db.QueryRow("SELECT COUNT(*) FROM acked").Scan(&n)
+	if n != 1 {
+		t.Fatalf("105 should remain in acked, got %d rows", n)
+	}
+}
+
+func TestUIDValidityChangeResets(t *testing.T) {
+	s := openTemp(t)
+	backlogAccount(t, s, false)
+	_ = s.EnsureFolderBaseline("work", "INBOX", 1, 100)
+	_ = s.Ack("work", "INBOX", 1, 105)
+	// Server reports a new UIDVALIDITY: state resets, re-baselines at new max.
+	if err := s.EnsureFolderBaseline("work", "INBOX", 2, 10); err != nil {
+		t.Fatalf("rebaseline: %v", err)
+	}
+	f, _ := s.floor("work", "INBOX")
+	if f != 10 {
+		t.Fatalf("floor should re-baseline to 10, got %d", f)
+	}
+	var n int
+	_ = s.db.QueryRow("SELECT COUNT(*) FROM acked").Scan(&n)
+	if n != 0 {
+		t.Fatalf("acked must reset on uidvalidity change, got %d", n)
+	}
+}